Metal oxide nanorod arrays on monolithic substrates

ABSTRACT

A metal oxide nanorod array structure according to embodiments disclosed herein includes a monolithic substrate having a surface and multiple channels, an interface layer bonded to the surface of the substrate, and a metal oxide nanorod array coupled to the substrate surface via the interface layer. The metal oxide can include ceria, zinc oxide, tin oxide, alumina, zirconia, cobalt oxide, and gallium oxide. The substrate can include a glass substrate, a plastic substrate, a silicon substrate, a ceramic monolith, and a stainless steel monolith. The ceramic can include cordierite, alumina, tin oxide, and titania. The nanorod array structure can include a perovskite shell, such as a lanthanum-based transition metal oxide, or a metal oxide shell, such as ceria, zinc oxide, tin oxide, alumina, zirconia, cobalt oxide, and gallium oxide, or a coating of metal particles, such as platinum, gold, palladium, rhodium, and ruthenium, over each metal oxide nanorod. Structures can be bonded to the surface of a substrate and resist erosion if exposed to high velocity flow rates.

RELATED APPLICATIONS

This application is the U.S. National Stage of International Application No. PCT/US2012/057974, filed on Sep. 28, 2012, published in English, which claims the benefit of U.S. Provisional Application No. 61/540,221, filed on Sep. 28, 2011. The entire teachings of the above applications are incorporated herein by reference.

GOVERNMENT SUPPORT

This invention was made with government support under contract No. DE-EE0000210 awarded by the Department of Energy. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

Ceramic monoliths are widely applied as catalyst supports as a result of their high inertness to undesired reactions, mechanical and thermal stability, and high flow rates along multiple channels. See Satterfield, C. W., Heterogeneous Catalysis in Industrial Practice, 2nd Edition, McGraw-Hill, Inc., New York, 1991. Monoliths, however, have relatively low surface area, and therefore usually require a high surface area porous alumina particulate washcoat to help increase their surface area before or while loading the catalytically active materials such as noble metal nanoparticles. Unfortunately, the intrinsically non-adherent particle-based washcoat can be easily subjected to erosion by high velocity flow rates.

SUMMARY OF THE INVENTION

Therefore, there is a need for a high surface area catalyst structure having an adherent layer bonded to the surface of the substrate.

A metal oxide nanorod array structure includes a monolithic substrate having a surface and multiple channels, an interface layer bonded to the surface of the substrate, and a metal oxide nanorod array coupled to the substrate surface via the interface layer.

In some embodiments, the metal oxide nanorod array can be vertically aligned relative to the surface of the substrate. In certain embodiments, the metal oxide can be titania (TiO₂), such as brookite titania. In other embodiments, the metal oxide can include one of the following: ceria (CeO₂), zinc oxide (ZnO), tin oxide (SnO₂), alumina (Al₂O₃), zirconia (ZrO₂), cobalt oxide (Co₃O₄), and gallium oxide (Ga₂O₃). The substrate can include at least one of the following: a glass substrate, a plastic substrate, a silicon substrate, a ceramic monolith, and a stainless steel monolith. In some embodiments, the ceramic can include one of the following: cordierite, alumina, tin oxide, and titania.

In certain embodiments, the nanorod array structure can further include a perovskite ABO₃ (A=La, Ba, Sr, and/or Pb, B=Co, Cr, Fe, Mn, or Ni) shell over each metal oxide nanorod, such as a lanthanum-based transition metal oxide, LaMO₃ (M=Co, Cr, Fe, Mn, and Ni), such as lanthanum strontium cobaltate and lanthanum strontium manganate. In other embodiments, the nanorod array structure can further include a metal oxide shell over each metal oxide nanorod, such as a metal oxide including one of the following: ceria (CeO₂), zinc oxide (ZnO), tin oxide (SnO₂), alumina (Al₂O₃), zirconia (ZrO₂), cobalt oxide (Co₃O₄), and gallium oxide (Ga₂O₃). In yet other embodiments, the nanorod array structure can further include an alkaline oxide shell over each metal oxide nanorod, such as a barium oxide and/or potassium oxide shell. In still other embodiments, the nanorod array structure can further include a coating of metal particles over each metal oxide nanorod, such as platinum, gold, palladium, rhodium, and ruthenium.

Another embodiment is a titania nanorod array structure made by the process of mixing an aqueous titanium trichloride solution with saturated sodium chloride to form a precursor solution, submerging a monolithic substrate having a surface and multiple channels in the precursor solution, and heating the substrate and the precursor solution at elevated, relative to ambient, pressure and temperature for a time sufficient to form brookite titania nanorod arrays coupled to the substrate surface via an interface layer, and vertically aligned relative to the substrate surface. The titanium trichloride solution can have a concentration in a range of between about 0.001 M and about 1.0 M, such as about 0.15 M. In some embodiments, heating the substrate and the precursor solution can be conducted in an autoclave, to a temperature in a range of between about 90° C. and about 240° C., such as about 180° C. The time for heating the substrate and the precursor solution can be in a range of between about 4 hours and about 24 hours. Some embodiments can further include radio frequency (RF) magnetron sputtering lanthanum strontium manganate (LSMO) nanofilm on the brookite titania nanorod arrays, thereby forming LSMO shells over the titania nanorod arrays. Other embodiments can further include submerging the substrate, including the brookite titania nanorod arrays, in a metal colloid solution, followed by drying the substrate, thereby forming metal-loaded brookite titania nanorod arrays on the substrate. The metal can include at least one of the following: platinum, gold, palladium, rhodium, and ruthenium. Yet other embodiments can further include submerging the monolithic substrate, including the brookite titania nanorod arrays, in a metal hydroxide solution, followed by drying the substrate, thereby forming metal-loaded brookite titania nanorod arrays on the substrate. Still other embodiments can further include submerging the monolithic substrate, including the brookite titania nanorod arrays, into a reactor containing H₂MCl₆ solution (M=Pt, Au, Pd, Rh, or Ru), and injecting a reductant solution or hydrogen gas into the reactor, followed by drying the substrate, thereby forming metal-loaded brookite titania nanorod arrays on the substrate. The reductant solution can include sodium borohydride.

Yet another embodiment is a nanorod array structure made by the process of submerging a monolithic substrate having a surface and multiple channels in a precursor solution, and heating the substrate and the precursor solution at elevated, relative to ambient, pressure and temperature for a time sufficient to form nanorod arrays coupled to the substrate surface via an interface layer. The precursor solution can be an aqueous metal salt solution, the metal including one of the following: aluminum, cerium, tin, zirconium, cobalt, and gallium. The interface layer can be made of the same materials as the nanorod arrays, or dissimilar materials, which can be grown prior to or during the nanorod growth process.

In still another embodiment, a method of making titania nanorod arrays includes mixing an aqueous titanium trichloride solution with saturated sodium chloride to form a precursor solution, submerging a monolithic substrate having a surface and multiple channels in the precursor solution, and heating the substrate and the precursor solution at elevated, relative to ambient, pressure and temperature for a time sufficient to form brookite titania nanorod arrays coupled to the substrate surface via an interface layer and vertically aligned relative to the substrate surface.

In yet another embodiment, a method of making zinc oxide nanorod arrays includes dip coating a zinc oxide nanoparticle seed film on a monolithic substrate having a surface and multiple channels, annealing the coated substrate, and submerging the coated substrate in a stirred aqueous solution of a zinc salt and a weak base, such as hexamethylenetetramine, for a time sufficient to form zinc oxide nanorod arrays coupled to the substrate surface via an interface layer. The coated substrate can be annealed at a temperature in a range of between about 200° C. and about 500° C. The time for submerging the coated substrate can be in a range of between about 0.1 hours and about 100 hours. Some embodiments can further include radio frequency (RF) magnetron sputtering cerium oxide nanofilms on the zinc oxide nanorod arrays, thereby forming zinc oxide/cerium oxide composite nanorod arrays. Other embodiments can further include submerging the substrate, including the zinc oxide nanorod arrays, in a metal colloid solution, followed by drying the substrate, thereby forming metal-loaded zinc oxide nanorod arrays on the substrate. Yet other embodiments can further include submerging the monolithic substrate, including the zinc oxide nanorod arrays, in a metal hydroxide solution, followed by drying the substrate, thereby forming metal-loaded zinc oxide nanorod arrays on the substrate. Still other embodiments can further include submerging the substrate, including the zinc oxide nanorod arrays, into a reactor containing H₂MCl₆ solution (M=Pt, Au, Pd, Rh, or Ru), and injecting a reductant solution or hydrogen gas into the reactor, followed by drying the substrate, thereby forming metal-loaded zinc oxide nanorod arrays on the substrate.

In still another embodiment, a method of making metal oxide nanowire arrays includes electrodepositing metal oxide onto a monolithic substrate having a surface and multiple channels from an electrolytic solution of metal-chloride or -nitrate and potassium chloride, the metal oxide nanowire arrays being coupled to the substrate surface via an interface layer. The concentration of metal-chloride or -nitrate and potassium chloride can each independently be in a range of between about 0.001 M and about 1.0 M. The electrodeposition can be conducted at a temperature in a range of between about 25° C. and about 90° C., such as about 70° C. The metal oxide can include one of the following: cerium oxide, zinc oxide, titania, zirconia, and tin oxide.

Embodiments of this invention have many advantages, including enabling catalytic structures with high surface area, high thermal stability, sulfur poisoning resistance, and multi-functional catalytic tailoring ability to be applied in various ways, such as being bonded via an adherent layer to the surface of a substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention.

FIGS. 1(a)-1(e) are photographs of: FIG. 1(a) and FIG. 1(b) top views of optical FIG. 1(a) and SEM FIG. 1(b) images of a honeycomb monolithic substrate, FIG. 1(c) side view SEM images of the honeycomb monolith and the corresponding EDX spectrum, FIG. 1(d) and FIG. 1(e) SEM images of TiO₂ nanorod arrays grown using 2 mL of TiCl₃, and 18 mL of NaCl-saturated DI water at 180° C. for 4 h on 3D honeycomb monolithic substrate.

FIGS. 2(a)-2(f) are photographs of SEM images of TiO₂ nanostructures grown on 3D honeycomb monolithic substrates using 2 mL of TiCl₃, and 18 mL of NaCl-saturated DI water at 180° C. for different times: FIG. 2(a) 1 h, FIG. 2(b) 1.5 h, FIG. 2(c) 2 h, FIGS. 2(e, f) 24 h. Single TiO₂ nanorods were also found on the substrate surface when the hydrothermal treatment time was 2 h, as indicated by the boxes in FIG. 2(c).

FIGS. 3(a)-3(f) are photographs of: SEM images of the dandelion-like TiO₂ nanorod clusters inside the 3D honeycomb monolithic substrate grown at 180° C. for 1.5 h in FIGS. 3(a) and 2 h in FIG. 3(b). Low magnification and (FIGS. 3d and 3e ) high magnification (FIG. 3d : body part; FIG. 3e : tip part) TEM images of a dandelion-like TiO₂ nanorod cluster grown inside the 3D honeycomb monolithic substrate at 180° C. for 2 h. FIG. 3(f) is an illustration of an electron diffraction (ED) pattern of the TiO₂ nanorod cluster. The distance between the lattice fringes in FIGS. 3(d)-3(e) is 3.4 nm, which can be assigned to brookite (111).

FIGS. 4(a)-4(c) are photographs of: FIG. 4(a) low-resolution TEM image of the TiO₂ nanorods grown inside the 3D honeycomb monolithic substrate at 180° C. for 4 h. Inset of FIG. 4(a) is the ED pattern of the grown TiO₂. FIGS. 4(b, c) are high-resolution TEM images of the body portion and top portion of one grown TiO₂ nanorod, respectively.

FIG. 5 shows superimposed graphs (a)-(e) of XRD patterns of (a) the honeycomb monolithic substrate, and the samples of TiO₂ nanorods grown inside the 3D honeycomb monolithic substrate prepared at 180° C. for (b) 1.5 h, (c) 2 h, (d) 4 h, and (e) 24 h, respectively.

FIGS. 6(a)-6(b) are graphs of TGA in FIG. 6(a) and DSC in FIG. 6(b) spectra of TiO₂ nanorods grown inside the 3D honeycomb monolithic substrates prepared at 180° C. for 1.5 h, 2 h, 4 h, and 24 h.

FIG. 7 is a graph of CO conversion (%) as a function of temperature for a TiO₂ nanorod-based 3D nanocatalyst, showing that the onset temperature for CO oxidation for the Pt—TiO₂/CH nanocatalyst is 50° C. less than that for the TiO₂/CH nanocatalyst, and the T₅₀ (temperature of 50% CO conversion) for Pt—TiO₂/CH nanocatalyst is 170° C., while the T₅₀ for the TiO₂/CH nanocatalyst is 350° C., as a result of Pt loading.

FIGS. 8A-8F are photographs of: FIG. 8(A) typical 3-D monolithic substrates used in the invention: cordierite honeycomb substrate, stainless steel substrate; FIG. 8(B) higher magnification SEM image of cordierite substrate channels; FIG. 8(C) higher magnification SEM image of stainless steel substrate channels; FIGS. 8(D-F) top view images of TiO₂, ZnO and Al₂O₃ arrays, respectively, grown on cordierite or stainless steel monolithic substrates.

FIGS. 9(a)-9(d) are photographs of SEM images of CeO₂ nanorods on a stainless steel substrate.

FIGS. 10A-10E are: FIG. 10A shows top-view FE-SEM images of TiO₂ nanorod arrays on the inside wall of a ceramic honeycomb monolithic substrate in a relatively high magnification; the inset of FIG. 10A is the EDS confirming the composition of TiO₂; FIG. 10B shows a cross-section view of the TiO₂ nanorods arrays on cordierite honeycomb monolithic substrate; FIG. 10C shows a TEM image of a typical TiO₂ nanorod; the inset is the selected diffraction pattern on the single nanorod; FIG. 10D shows a HRTEM image of TiO₂ nanorod; FIG. 10E shows X-ray diffraction (XRD) patterns of blank cordierite monolithic substrate and TiO₂ nanorod arrays on cordierite monolithic substrate. FIG. 10B shows the TiO₂ nanorod arrays 10, the TiO₂ interface layer 20, and the cordierite monolithic substrate 30.

FIGS. 11A-11F are EM images of the prepared samples: FIG. 11A shows a top view of CeO₂/ZnO nanofilm-nanorod arrays in low magnification; FIG. 11B shows a cross section of CeO₂/ZnO nanofilm-nanorod arrays; FIG. 11C shows a bright field TEM image of a typical CeO₂/ZnO core-shell nanorod; FIG. 11D shows a corresponding dark field image of CeO₂/ZnO core-shell nanorod in FIG. 11C; FIG. 11E shows a selective area diffraction pattern of CeO₂/ZnO core-shell nanorods; FIG. 11F shows an EDS spectrum of CeO₂/ZnO core-shell nanorods.

FIGS. 12A-12D are SEM images of Al₂O₃—TiO₂ heterogeneous nanorod arrays: FIG. 12A shows a top view of Al₂O₃—TiO₂ heterogeneous nanorod arrays; FIG. 12B shows typical morphology of Al₂O₃ nanorods; FIG. 12C shows a cross-sectional view of Al₂O₃—TiO₂ heterogeneous nanorod arrays; FIG. 12D shows element mapping across part of the Al₂O₃—TiO₂ heterogeneous nanorod arrays shown in FIG. 12C.

FIGS. 13A-13E are electron microscopy images of TiO₂/LSMO core-shell nanorod arrays: FIG. 13A large area scanning electron microscopy (SEM) image of TiO₂/LSMO core-shell nanorod array, FIG. 13B cross-sectional view SEM image of TiO₂/LSMO nanorod array, FIG. 13C transmission electron microscopy (TEM) image of TiO₂/LSMO core-shell nanorods, FIG. 13D high resolution TEM image of TiO₂/LSMO core-shell nanorods, FIG. 13E EDS spectrum of TiO₂/LSMO nanorods under TEM.

FIGS. 14A-14D are TEM images of TiO₂/Pt composite nanorods: FIG. 14A bright field image of TiO₂ nanorods; FIG. 14B dark field image of the same TiO₂/Pt composite nanorods; FIG. 14C TEM images of a typical TiO₂/Pt composite nanorod in a higher magnification; FIG. 14D high-resolution TEM images of the TiO₂—Pt interface.

FIG. 15 shows schematic illustrations of 3-D configurated nano-array catalysts, and typical metal oxide nano-array based monolithic catalysts. FIG. 15a . Schematic nano-array assembly process on typical ceramic and stainless steel monolith substrates. FIG. 15b , Selected nano-array based monolithic catalysts (left to right: ZnO/stainless steel, TiO₂/cordierite, CeO₂/cordierite, and Co₃O₄/cordierite). SEM images of metal oxide nano-arrays on monolith substrates: FIG. 15c , Low magnification view of cordierite monolithic channels grown with ZnO nanowire arrays; inset: Zn and Al element map of a single monolith channel from EDXS. FIGS. 15d, and 15e , top and cross-sectional views of Co₃O₄ porous nanowire arrays on cordierite monolith. FIGS. 15f and 15j, 15g and 15k, 15h and 15l, and 15i and 15m , are, respectively, ZnO nanorod arrays, TiO₂ nanorod arrays, CeO₂ nanotube arrays, Co₃O₄ porous nanowire arrays on cordierite monolith and their corresponding TEM images. Scale bar in all the insets is 2 nm.

FIG. 16 shows the thermal and mechanical robustness of metal oxide nano-array based monolithic catalysts in ambient atmosphere. Time dependent Thermo-Gravimetric Analysis (TGA), Brunauer-Emmett-Teller (BET) specific surface area, morphology and crystal phase change, and time dependent morphology and weight loss at constant air flow of 50 L/min. Plots: FIG. 16 a. 1 μm ZnO nanorod array grown on stainless steel monolith. FIG. 16 b. 1 μm TiO₂ nanorod array grown on cordierite monolith. FIG. 16 c. 8 μm Co₃O₄ porous nanowire array grown on cordierite monolith. Scale bar: 100 nm for SEM insets in all thermal and mechanical tests in FIG. 16 b; 1 μm for SEM insets in the mechanical tests in FIGS. 16a and 16 c.

FIG. 17 shows CO oxidation over Pt-loaded metal oxide nano-array based monolithic catalysts. Low magnification TEM images of individual metal oxide nanostructure loaded with Pt nanoparticles and its high magnification HRTEM lattice image (inset): FIG. 17a ) ZnO nanorod; FIG. 17b ) TiO₂ nanorod; FIG. 17c ) CeO₂ nanotube; and FIG. 17d ) Co₃O₄ nanowire. Scale bars in all the insets are 2 nm. FIG. 17e ) CO oxidation conversion as a function of temperature over Pt-loaded (CeO₂, ZnO, Co₃O₄, and TiO₂) nano-array monolithic catalysts; FIG. 17f ) Catalytic stability of Pt-loaded (CeO₂, ZnO, Co₃O₄, and TiO₂) nano-array monolithic catalysts; FIG. 17g ) Materials utilization efficiency of nano-array catalysts and open literature reported classical-powder washcoated catalysts; FIG. 17h ) a schematic model of nano-array catalysts and traditional nano-powder catalysts.

FIG. 18 shows CO oxidation over well-defined Pt—ZnO nano-array based monolithic catalysts. SEM images of well-defined ZnO nano-array: FIG. 18a ) ZnO Plates; FIG. 18b ) 1 μm ZnO nanorods; FIG. 18c ) 5 μm ZnO nanorods; FIG. 18d ) 5 μm ZnO washcoated powders; FIG. 18e ) CO oxidation conversion as a function of temperature over 1 μm and 5 μM long Pt-loaded ZnO nanorod array and washcoated powder catalysts. FIG. 18f ) CO oxidation conversion as a function of temperature over Pt—ZnO nanorod arrays and Pt—ZnO nanoplate arrays. The 2 nm Pt nanoparticles were loaded on ZnO nano-array monoliths. Scale bar in all the insets is 1 μm.

FIG. 19 shows SEM, EDXS, TEM and SAED analyses of single-component nanostructure arrays grown on cordierite monoliths: ZnO nanowire and nanorod array; TiO₂ nanorod array; CeO₂ nanotube array; and Co₃O₄ porous nanowire array.

FIG. 20 shows SEM and EDXS analyses of single-component nanostructure array grown on stainless steel monoliths: Co₃O₄ porous nanowire array and ZnO nanorod array.

FIG. 21 shows TEM and EDXS analyses of Pt/Metal oxide nanostructures on cordierite monoliths: Pt/TiO₂ nanorods; Pt/ZnO nanorods; Pt/CeO₂ nanotubes; and Pt/Co₃O₄ porous nanowires.

FIG. 22 shows XRD spectra of metal-oxide nanorod arrays on cordierite monolith substrates: TiO₂ and Pt/TiO₂ nanorod arrays; ZnO and Pt/ZnO nanorod/nanowire arrays; CeO₂ nanotube arrays; and Co₃O₄ and Pt/Co₃O₄ porous nanowire arrays.

FIG. 23 shows electron microscopy images and spectra of metal oxide-metal oxide binary nanostructure array on monolith: ZnO/LSCO decorated nanorods; ZnO/CeO₂ decorated nanorods; and TEM image and corresponding EDS line analysis on ZnO/CeO₂ core-shell nanorod.

FIG. 24 shows a set of TEM images of typical ZnO/LSCO/Pt nanorods collected from the corresponding monolithic nano-array catalysts (top panel); and typical EDXS spectrum (bottom panel) of the ZnO/LSCO/Pt composite nanorod.

FIG. 25 shows thermal analyses (TGA, DSC) spectra and the corresponding SEM images after TGA testing of Ce nanotubes array on cordierite monolith at 400° C., 500° C., and 1000° C.

FIG. 26 shows thermal analyses (TGA and DSC) spectra of bare cordierite monolith substrate.

FIG. 27 shows mechanical stability test using static or pulsatile flow. FIG. 27a ) schematic setup of high velocity air flow (static 50 L/min or pulsatile flow) stability test on nano-array based monolithic catalysts; FIG. 27b ) Time dependent morphology and weight loss of 1 μm CeO₂ nanotube array on cordierite monolith at a constant air flow of 50 L/min.

FIG. 28 shows TGA, DSC spectra and corresponding SEM image after testing of one-dimensional nanostructure grown on cordierite monolith: TiO₂ nanorod array; ZnO nanowires array; and Co₃O₄ porous nanowires array.

FIG. 29 shows the pore size distribution of Co₃O₄ porous nanowire arrays on cordierite monolith after 0 h, 24 h, 48 h, 72 h, and 100 h of annealing time at 800° C.

FIG. 30 shows TPR-H₂ spectra and corresponding SEM images after testing: FIG. 30a , TPR-H₂ spectra of bare cordierite, CeO₂ nanotube on cordierite, ZnO nanowires on cordierite, TiO₂ nanorod on cordierite. FIG. 30b , TPR-H₂ spectra of Co₃O₄ porous nanowire on cordierite. FIG. 30c , CeO₂ nanotubes array after TPR test; Co₃O₄ porous nanowire array after test; TiO₂ nanorod array after TPR test; and ZnO nanowires array after test.

FIG. 31 shows XRD spectra of Co₃O₄ porous nanowire array on monolith substrate before and after TPR-H₂.

FIG. 32 shows pulsatile flow flushing flow-rate vs. cycles (top panel) and corresponding SEM images after testing (bottom panel): ZnO nanowires array; TiO₂ nanorod array after TPR test; Co₃O₄ porous nanowire array after TPR test; and CeO₂ nanotubes array after TPR test.

FIG. 33 shows SEM and TEM images of nanostructure array catalysts after CO oxidation light-off and catalytic stability test: Pt/ZnO nanorod array catalysts; Pt/TiO₂ nanorod array catalysts; Pt/Co₃O₄ porous nanowire array catalysts; inset: corresponding TEM image; and Pt/CeO₂ nanotubes array catalysts; inset: corresponding TEM image.

FIG. 34 shows TEM images of nanostructure array catalysts before (left) and after (right) CO oxidation light-off and catalytic stability test: Pt/ZnO nanorod array catalysts; Pt/TiO₂ nanorod array catalysts; Pt/Co₃O₄ porous nanowire array catalysts; and Pt/CeO₂ nanotubes array catalysts. Axis of inset spectra: Particle diameter (nm). The diameter range was determined by covering up 70% of nanoparticles.

FIG. 35 is a graph of CO oxidation of metal oxide nano-arrays on cordierite monolith.

FIG. 36 shows TEM images of Pt/ZnO nanoplate loaded on cordierite monolith; top left panel: A few Pt/ZnO nanoplates; top right panel: Electron diffraction pattern of a typical ZnO nanoplate; bottom left panel: A typical TEM image of ZnO hexagonal nanoplate; bottom right panel: An HRTEM image of Pt nanoparticles loaded on ZnO nanoplate.

FIG. 37 shows SEM and TEM images of Pt/ZnO powder loaded on cordierite monolith (control sample): top left panel: large area Pt/ZnO powder on monolith; top right panel: top-view of Pt/ZnO powder catalysts on a higher magnification (upper portion) and cross-sectional view of Pt/ZnO powder catalysts on monolith (lower portion); bottom left panel: TEM image of Pt/ZnO powder; and bottom right panel: HRTEM image of Pt nanoparticles loaded on ZnO particle.

DETAILED DESCRIPTION OF THE INVENTION

A description of example embodiments of the invention follows.

A cost-effective solution synthesis of commercial scale single crystalline titanium dioxide (TiO₂) nanorod arrays has been successfully achieved on the inner walls of cordierite monolithic substrates having a surface area and multiple channels, which may provide a new class of nanostructured hybrid supports or catalysts for catalysis and energy applications. The morphological evolution has been observed from dandelion TiO₂ nanorod clusters to vertically aligned nanorod arrays while increasing the solution treatment time. A heterogeneous nucleation-dissolution-crystallization mechanism is proposed for the growth of the brookite structured TiO₂ nanorod arrays on the inner walls of honeycomb monoliths. The brookite structured TiO₂ nanorod arrays have a very high surface area close to about 700 m²/g, with an overall specific surface area in the hybrid nanostructured monolith increased by 1.8 and 2.5 times after coating the TiO₂ nanorods for 4 hours and 24 hours, respectively. The thermal stability tests using Differential Scanning calorimetry (DSC) and Thermo gravimetric analysis (TGA) suggest that the TiO₂ nanorods coated hybrid monoliths are very stable.

Ceramic monoliths are widely applied as catalyst supports as a result of their high inertness to undesired reactions, and mechanical and thermal stability. See Satterfield, C. W., Heterogeneous Catalysis in Industrial Practice, 2nd Edition, McGraw-Hill, Inc., New York, 1991 (hereinafter “Satterfield”). Monoliths are structures that contain various types of interconnected or separated channels in a single block of material, thereby maximizing the surface area-to-volume ratio. Monoliths, however, have relatively low-surface-area and, therefore, usually require a high surface area porous alumina particulate washcoat to help increase their surface area before or while loading the catalytically active materials such as noble metal nanoparticles. The intrinsically non-adherent, particle-based washcoat may be easily subjected to high velocity flow erosion.

As an important catalyst support, titania (TiO₂) has been extensively studied. V₂O₅/TiO₂ has been suggested as one of better catalysts for NO reduction under NH₃ for industrial power plants. Id. The latest research results demonstrate that the addition of TiO₂ to the conventional alumina washcoat support increases the NO_(x) absorber catalysts' sulfur tolerance, which is useful to enhance the catalysts' performance and life-time. See Despres, J.; Koebel, M.; Krocher, O.; Elsener, M.; Wokaun, A. Storage of NO₂ on BaO/TiO₂ and the influence of NO. Appl. Catal. B 2003, 43, 389-395; and Andonova, S. M.; Sentürk, G. S.; Ozensoy, E. Fine-Tuning the Dispersion and the Mobility of BaO Domains on NO_(x) Storage Materials via TiO₂ Anchoring Sites. J. Phys. Chem. C 2010, 114, 17003-17016. TiO₂ has three polymorphs of different symmetries, including anatase, rutile and brookite, all of which can be described in terms of distorted TiO₆ octahedra with different symmetries or arrangements. See Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69-96; and Gateshki, M.; Yin, S.; Ren, Y.; Petkov, V. Titania Polymorphs by Soft Chemistry: Is There a Common Structural Pattern? Chem. Mater. 2007, 19, 2512-2518. Anatase and rutile structures have been extensively studied, while brookite structure is much less studied due to its absence in natural TiO₂ phases. Anatase TiO₂ nanoparticles can be achieved with 200-300 m²/g high surface area by hydrolysis of an organic titanium compound such as tetraisopropyl titania in aqueous solution. See Satterfield. Brookite or rutile structured TiO₂ are normally believed to have low surface area compared to anatase TiO₂, despite the higher thermal stability, due to the difficulty of synthesizing fine structures.

In addition to nanoparticles, TiO₂ in the form of nanorods, nanowires, and nanotubes have attracted extensive attention in the past two decades as a result of their unique shape-dependent electronic and optical properties, as well as their widespread applications in batteries, solar cells, and sensors. See Alivisatos, A. P. Semiconductor Clusters, Nanocrystals, and Quantum Dots. Science 1996, 271, 933-937; Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Nanobelts of Semiconducting Oxides. Science 2001, 291, 1947-1949; Yang, P. D.; Yan, H. Q.; Mao, S.; Russo, R.; Johnson, J.; Saykally, R.; Morris, N.; Pham, J.; He, R. R.; Cho, H. J. Controlled Growth of ZnO Nanowires and Their Optical Properties. Adv. Funct. Mater. 2002, 12, 323-331; Kavan, L.; Gratzel, M.; Gilbert, S. E.; Klemenz, C.; Scheel, H. J. Electrochemical and Photoelectrochemical Investigation of Single-Crystal Anatase. J. Am. Chem. Soc. 1996, 118, 6716-6723; Oregan, B.; Gratzel, M. A Low-Cost, High-Efficiency solar Cell Based on Dye-Sensitized Colloidal TiO₂ Films Nature 1991, 353, 737-740; Wu, N.; Wang, S.; Rusakova, I. A. Inhibition of Crystallite Growth in the Sol-Gel Synthesis of Nanocrystalline Metal Oxides. Science 1999, 285, 1375-1377. However, crystal structure and symmetry of TiO₂ make the growth of oriented anisotropic single-crystalline TiO₂ films or nanorod arrays very difficult. See Liu, B.; Aydil, E. S. Growth of Oriented Single-Crystalline Rutile TiO₂ Nanorods on Transparent Conducting Substrates for Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2009, 131, 3985-3990. To date, there are only a few reports that describe heterogeneous growth of oriented single-crystalline TiO₂ nanorods or nanowires, but most growths are restricted to fluorine-doped tin oxide (FTO) substrates. See Id.; and Feng, X.; Shankar, K.; Varghese, O. K.; Paulose, M.; Latempa, T. J.; Grimes, C. A. Vertically Aligned Single Crystal TiO₂ Nanowire Arrays Grown Directly on Transparent Conducting Oxide Coated Glass: Synthesis Details and Applications. Nano Lett. 2008, 8, 3781-3786. In addition, most heterogeneous growths rely on vapor phase technique utilizing heterogeneous nucleation at very high growth temperatures. See Weng, C. C.; Hsu, K. F.; Wei, K. H. Synthesis of Arrayed, TiO₂ Needlelike Nanostructures via a Polystyrene-block-poly(4-vinylpyridine) Diblock Copolymer Template. Chem. Mater. 2004, 16, 4080-4086; and Chen, C. A.; Chen, Y. M.; Korotcov, A.; Huang, Y. S.; Tsai, D. S.; Tiong, K. K. Growth and characterization of well-aligned densely-packed rutile TiO₂ nanocrystals on sapphire substrates via metal-organic chemical vapor deposition. Nanotechnology 2008, 19, 075611. Compared to vapor deposition techniques, solution-based methods are more suitable for low-cost commercial scale-up production.

A hydrothermal synthesis procedure is described below which allows the one-step, large scale anisotropic growth of aligned, crystalline brookite TiO₂ nanorod arrays adhered on the channel walls of 3D honeycomb monoliths. With increase of hydrothermal treatment time, the brookite nanostructure morphology was identified to evolve from dispersive dandelion TiO₂ nanorod clusters to vertically aligned nanorod arrays fully covered on the monolith substrate. The scale-up synthesis of TiO₂ nanorod arrays inside the 3D honeycomb monoliths enables a new type of 3D hybrid nanostructured catalyst support, which can be directly applied to various applications, including catalyst supports, particulate matter filters, and environmental remediation and separation devices.

In some embodiments, the metal oxide nanorod array can be vertically aligned relative to the surface of the substrate. In certain embodiments, the metal oxide can be titania (TiO₂), such as brookite titania. In other embodiments, the metal oxide can include one of the following: ceria (CeO₂), zinc oxide (ZnO), tin oxide (SnO₂), alumina (Al₂O₃), zirconia (ZrO₂), cobalt oxide (Co₃O₄), and gallium oxide (Ga₂O₃). The substrate can include at least one of the following: a glass substrate, a plastic substrate, a silicon substrate, a ceramic monolith, and a stainless steel monolith. In some embodiments, the ceramic can include one of the following: cordierite, alumina, tin oxide, and titania.

In certain embodiments, the nanorod array structure can further include a perovskite ABO₃ (A=La, Ba, Sr, and/or Pb, B=Co, Cr, Fe, Mn, or Ni) shell over each metal oxide nanorod, such as a lanthanum-based transition metal oxide, LaMO₃ (M=Co, Cr, Fe, Mn, and Ni), such as lanthanum strontium cobaltate and lanthanum strontium manganate. In other embodiments, the nanorod array structure can further include a metal oxide shell over each metal oxide nanorod, such as a metal oxide including one of the following: ceria (CeO₂), zinc oxide (ZnO), tin oxide (SnO₂), alumina (Al₂O₃), zirconia (ZrO₂), cobalt oxide (Co₃O₄), and gallium oxide (Ga₂O₃). In yet other embodiments, the nanorod array structure can further include an alkaline oxide shell over each metal oxide nanorod, such as a barium oxide and/or potassium oxide shell. In still other embodiments, the nanorod array structure can further include a coating of metal particles over each metal oxide nanorod, such as platinum, gold, palladium, rhodium, and ruthenium.

Another embodiment is a titania nanorod array structure made by the process of mixing an aqueous titanium trichloride solution with saturated sodium chloride to form a precursor solution, submerging a monolithic substrate having a surface and multiple channels in the precursor solution, and heating the substrate and the precursor solution at elevated, relative to ambient, pressure and temperature for a time sufficient to form brookite titania nanorod arrays coupled to the substrate surface via an interface layer, and vertically aligned relative to the substrate surface. The titanium trichloride solution can have a concentration in a range of between about 0.001 M and about 1.0 M, such as about 0.15 M. In some embodiments, heating the substrate and the precursor solution can be conducted in an autoclave, to a temperature in a range of between about 90° C. and about 240° C., such as about 180° C. The time for heating the substrate and the precursor solution can be in a range of between about 4 hours and about 24 hours. Unless otherwise indicated, all ranges defined herein include the endpoints, and all numerical values of the endpoints can be plus or minus 20%. Some embodiments can further include radio frequency (RF) magnetron sputtering lanthanum strontium manganate (LSMO) nanofilm on the brookite titania nanorod arrays, thereby forming LSMO shells over the titania nanorod arrays. Other embodiments can further include submerging the substrate, including the brookite titania nanorod arrays, in a metal colloid solution, followed by drying the substrate, thereby forming metal-loaded brookite titania nanorod arrays on the substrate. The metal can include at least one of the following: platinum, gold, palladium, rhodium, and ruthenium. Yet other embodiments can further include submerging the monolithic substrate, including the brookite titania nanorod arrays, in a metal hydroxide solution, followed by drying the substrate, thereby forming metal-loaded brookite titania nanorod arrays on the substrate. Still other embodiments can further include submerging the monolithic substrate, including the brookite titania nanorod arrays, into a reactor containing H₂MCl₆ solution (M=Pt, Au, Pd, Rh, or Ru), and injecting a reductant solution or hydrogen gas into the reactor, followed by drying the substrate, thereby forming metal-loaded brookite titania nanorod arrays on the substrate. The reductant solution can include sodium borohydride.

Yet another embodiment is a nanorod array structure made by the process of submerging a monolithic substrate having a surface and multiple channels in a precursor solution, and heating the substrate and the precursor solution at elevated, relative to ambient, pressure and temperature for a time sufficient to form nanorod arrays coupled to the substrate surface via an interface layer. The precursor solution can be an aqueous metal salt solution, the metal including one of the following: aluminum, cerium, tin, zirconium, cobalt, and gallium. The interface layer can be made of the same materials as the nanorod arrays, or dissimilar materials, which can be grown prior to or during the nanorod growth process.

In still another embodiment, a method of making titania nanorod arrays includes mixing an aqueous titanium trichloride solution with saturated sodium chloride to form a precursor solution, submerging a monolithic substrate having a surface and multiple channels in the precursor solution, and heating the substrate and the precursor solution at elevated, relative to ambient, pressure and temperature for a time sufficient to form brookite titania nanorod arrays coupled to the substrate surface via an interface layer and vertically aligned relative to the substrate surface.

In yet another embodiment, a method of making zinc oxide nanorod arrays includes dip coating a zinc oxide nanoparticle seed film on a monolithic substrate having a surface and multiple channels, annealing the coated substrate, and submerging the coated substrate in a stirred aqueous solution of a zinc salt and a weak base for a time sufficient to form zinc oxide nanorod arrays coupled to the substrate surface via an interface layer. The coated substrate can be annealed at a temperature in a range of between about 200° C. and about 500° C. The time for submerging the coated substrate can be in a range of between about 0.1 hours and about 100 hours. Some embodiments can further include radio frequency (RF) magnetron sputtering cerium oxide nanofilms on the zinc oxide nanorod arrays, thereby forming zinc oxide/cerium oxide composite nanorod arrays. Other embodiments can further include submerging the substrate, including the zinc oxide nanorod arrays, in a metal colloid solution, followed by drying the substrate, thereby forming metal-loaded zinc oxide nanorod arrays on the substrate. Yet other embodiments can further include submerging the monolithic substrate, including the zinc oxide nanorod arrays, in a metal hydroxide solution, followed by drying the substrate, thereby forming metal-loaded zinc oxide nanorod arrays on the substrate. Still other embodiments can further include submerging the substrate, including the zinc oxide nanorod arrays, into a reactor containing H₂MCl₆ solution (M=Pt, Au, Pd, Rh, or Ru), and injecting a reductant solution or hydrogen gas into the reactor, followed by drying the substrate, thereby forming metal-loaded zinc oxide nanorod arrays on the substrate.

FIG. 1 shows a set of optical and scanning electron microscopy (SEM) images of 3D cordierite monolith and the TiO₂ nanorod arrays synthesized inside the monolith channels by hydrothermal treatment of aqueous titanium trichloride (TiCl₃) solutions with saturated sodium chloride (NaCl) at 180° C. for 4 h. FIGS. 1(a) and 1(b) display the top view cross section of monolith. The monolith has a cell density of 400 cells per square inch with a web thickness of about 0.1 mm. FIG. 1(c) shows the side view SEM images of the monolith. It can be clearly seen that the wall surface of the monolith is not a flat surface, but includes a certain porosity. The EDS analysis reveals the monolith to be made of Si, Mg, Al, and Nb. The grown TiO₂ nanorods are highly aligned and densely packed on the monolith substrate with a diameter of about 50 nm and a length of about 250 nm (FIGS. 1(c) and 1(d)).

To investigate the growth behavior of TiO₂ nanorods inside the 3D cordierite monolith, hydrothermal treatment times of 1, 1.5, 2, 4 and 24 hours (h) were applied and the evolution of TiO₂ nanostructures was examined by an array of SEM imaging analysis (FIG. 2). With 1 h growth time, no obvious TiO₂ nanostructures grew on the wall surface of monolith, as indicated in the SEM image in FIG. 2(a). Prolonging the reaction time to 1.5 h, the TiO₂ nanorod clusters were grown on the wall surface with a dandelion-like morphology with diameters in the range of 250-350 nm (FIG. 2(b)). The TiO₂ nanorod clusters included many close-packed TiO₂ nanorods grown in a radially outward direction. Further prolonging the reaction time to 2 h, the TiO₂ nanorod clusters grew even bigger with diameters in the range of 400-500 nm (FIG. 2(c)). At high magnification (FIG. 2(c)), some nanorods in the TiO₂ nanorod clusters grew longer than others and clearly out of the dandelion-like sphere surface. More interestingly, some individual TiO₂ nanorods directly grew out of the substrate surface as indicated by the boxes in FIG. 2(c). When the reaction time increased to 4 h, highly aligned TiO₂ nanorod arrays grew on the cordierite substrate surface. FIG. 2(d) shows the SEM image taken on an extremely rough surface of the substrate. It certainly confirmed the TiO₂ nanorods grew everywhere on the monolith surface, even on the porous surface in the substrate. When the reaction time was further increased to 24 h, no obvious morphology change was observed on the TiO₂ nanorod arrays.

As shown by the low magnification SEM images in FIGS. 3(a) and 3(b), the TiO₂ nanostructures dispersively grew on the monolith after hydrothermal treatment for 1.5 and 2 h (FIGS. 3(a) and 3(b)), which confirmed that the dandelion TiO₂ nanorod clusters were grown all over the substrate with a very uniform distribution. The TiO₂ nanorods assembly and morphology were further characterized by transmission electron microscopy (TEM). FIG. 3(c) shows the low-resolution TEM image of a typical TiO₂ nanorod cluster grown for 2 h. The dandelion-like sphere morphology and an out-grown TiO₂ nanorod (identified in the rectangular box in FIG. 3(c)) were clearly identified. The diameter of the single TiO₂ nanorod is about 8-10 nm. The high-resolution TEM images in FIGS. 3(d) and 3(e) revealed the body and top portions of the TiO₂ nanorods, respectively. A clear set of lattice fringes was revealed with 0.34 nm spacing, suggesting the single crystalline nature of the grown TiO₂ nanorods. The electron diffraction (ED) pattern (FIG. 3(f)) in a distinct ring form confirms the brookite structure of grown TiO₂ nanorods after 2-hour hydrothermal treatment. The lattice fringes spacing of 0.34 nm can be assigned to the interplanar distance of single crystalline brookite TiO₂ (111).

FIG. 4(a) shows the low-resolution TEM image of TiO₂ nanorods grown inside the 3D honeycomb monolith for 4 h. Compared with the TEM image of FIG. 3(c), no TiO₂ nanorod clusters were found when the hydrothermal treatment time is applied for 4 h, indicating the conversion of dandelion-like TiO₂ nanorod clusters into aligned TiO₂ nanorod arrays. FIGS. 4(b) and 4(c) show the high-resolution TEM images of the body and top portions of a typical TiO₂ nanorod. The atomic planes corresponding to (111) are clearly visible with the same interplanar distance of 0.34 nm, suggesting the single crystalline nature of the brookite TiO₂ nanorods grown after hydrothermal treatment for 4 h. The distinct ring-form ED pattern (inset of FIG. 4(a)) further confirmed the good crystallinility and brookite structure of TiO₂ nanorods.

The brookite crystal structure of the grown TiO₂ nanorods at different hydrothermal treatment times was further confirmed by X-ray diffraction (XRD) analysis. FIG. 5 shows the XRD patterns of the bare honeycomb monolith and the synthesized TiO₂ nanostructures inside the honeycomb monolith. Since the honeycomb monolith is a mixed oxide cordierite of SiO₂, MgO, Al₂O₃ and Nb₂O₅, some characteristic peaks of the honeycomb monolith overlap with the diffraction peaks of brookite TiO₂. As shown in FIG. 5, when the hydrothermal time is less than 4 h (spectra (b) and (c)), no significant TiO₂ diffraction peaks appeared in the corresponding XRD pattern, although the TiO₂ nanorod clusters were obtained on the substrate surface according to the SEM imaging (FIGS. 3a and 3b ). This might be due to their dispersive distribution (FIGS. 2b and 2c )), and the extremely strong intensity from the exposed monolith substrate. However, with increasing hydrothermal time, the weakening intensities from the substrate is accompanied by the enhanced intensities from the brookite TiO₂. As the hydrothermal treatment time increased to 4 h, two weak peaks appeared in diffraction spectrum d in FIG. 5 at about 25.4° and 25.7° corresponding to brookite (012) and (111), respectively (JCPDS No. 00-029-1360), which do not overlap with any diffraction peaks of the substrate. As the hydrothermal treatment time further increased to 24 h, the peaks in brookite (012) and (111) in diffraction spectrum e shown in FIG. 5 became more significant with higher intensities than those of the substrate, suggesting a complete coverage of TiO₂ nanorod arrays on the monolith as well as the further improved crystallinity.

Brookite TiO₂ with different morphologies has been reported in the literature. For example, Cozzoli et al. synthesized anisotropically shaped brookite TiO₂ nanocrystals using a surfactant-assisted nonaqueous strategy. See Buonsanti, R.; Grillo, V.; Carlino, E.; Giannini, C.; Kipp, T.; Cingolani, R.; Cozzoli, P. D. Nonhydrolytic Synthesis of High-Quality Anisotropically Shaped Brookite TiO₂ Nanocrystals. J. Am. Chem. Soc. 2008, 130, 11223-11233. A self-regulated phase-switching seed-catalyzed mechanism was proposed to explain the heterogeneous nucleation of brookite TiO₂ on the initially generated c-axis-elongated anatase TiO₂ and the anatase-to-brookite conversion. Sun et al. prepared high-quality brookite TiO₂ flowers in the presence of NaOH by a solution chemistry technique. See Hu, W.; Li, L.; Li, G.; Tang, C.; Sun, L. High-Quality Brookite TiO₂ Flowers: Synthesis, Characterization, and Dielectric Performance. Cryst. Growth Des. 2009, 9, 3676-3682. Hu et al. proposed three steps for the synthesis of flower-like brookite TiO₂: (1) the transformation of layer structured titania into brookite nanoparticles, (2) the evolution of brookite particles to the spindle-like shape, and (3) the assembly of these spindle-like particles into flower-like TiO₂. Without wishing to be bound by any particular theory, the formation of the aligned TiO₂ nanorod arrays on 3D honeycomb monoliths according to example embodiments disclosed herein can be different from the immediately aforementioned mechanisms.

According to the time-dependent morphology evolution evidence shown here (FIGS. 2-4), the formation of multi-nanostructures and morphology evolution from dandelion TiO₂ nanorod clusters to aligned nanorod arrays can be expressed as a heterogeneous nucleation-dissolution-crystallization mechanism. Generally, titanium tetrachloride (TiCl₄) or titanium alkoxides (Ti(OR)₄) are used as a titanium source to synthesize TiO₂. However, the reaction rate of Ti(IV) in forming TiO₂ is too fast. A high degree of supersaturation is obtained due to the production of many nuclei in the solution, which accelerates the homogeneous nucleation to produce TiO₂ nanoparticles. In this application, a low degree of supersaturation was achieved by using TiCl₃ as the titanium precursor due to the slow oxidation of Ti(III) by dissolved oxygen. The low degree of supersaturation promotes the heterogeneous nucleation of TiO₂ on the wall surface of the honeycomb monolith. See Hosono, E.; Fujihara, S.; Kakiuchi, K.; Imai, H. Growth of Submicrometer-Scale Rectangular Parallelepiped Rutile TiO₂ Films in Aqueous TiCl₃ Solutions under Hydrothermal Conditions. J. Am. Chem. Soc. 2004, 126, 7790-7791; and Hosono, E.; Fujihara, S.; Kakiuchi, K.; Imai, H.; Honma, I.; Zhou, H. One-Step Synthesis of Nano-Micro Chestnut TiO₂ with Rutile Nanopins on the Microanatase Octahedron. ACS Nano 2007, 1, 273-278. Furthermore, a high concentration of chlorine ions in the solution helps the formation of brookite TiO₂, and also avoids the conversion of brookite into rutile after prolonging the hydrothermal treatment time. See Pottier, A.; Chanéac, C.; Tronc, E.; Mazeroles, L.; Jolivet, J. Synthesis of Brookite TiO₂ Nanoparticles by Thermolysis of TiCl₄ in Strongly Acidic Aqueous Media. J. Mater. Chem. 2001, 11, 1116-1121. The brookite structure was obtained at the very beginning of the reaction (FIG. 3) and prolonging the reaction time from 4 h to 24 h did not change the brookite structure but improved its crystallinity, as shown in FIG. 5. With the increase in hydrothermal time, the hydrothermal conditions became more acidic, which might make the not-well-crystallized TiO₂ nanorods dissolve and the dissolved TiO₂ in the solution might nucleate onto the existing nanorods, which is confirmed by the change of TiO₂ morphology from closely packed nanorod clusters to sparsely aligned arrays.

To look into the thermal stability of TiO₂ nanorod arrays coated onto hybrid monolith substrates, thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out in ambient condition. The TGA and DSC curves are plotted in FIG. 6. Blank honeycomb monolith substrate is very stable over the whole temperature range that was investigated. During the thermal analysis, different samples exhibit different degrees of weight loss. TiO₂ samples grown for 1.5 and 2 h showed a similar weight loss with less than 1% in total. That is possibly due to their similar cluster morphology and relatively low density distribution. TiO₂ nanorod arrays grown for 4 h had less than 2% weight loss, while the TiO₂ nanorod arrays grown for 24 h had the highest weight loss of 3.5%. However, the highest weight loss was just about 3%, which suggests that TiO₂ nanorods arrays on the honeycomb monolith substrates are very stable in the temperature range of 20-800° C. The weight loss in the temperature range of 40-250° C., about 2% for the sample grown for 24 h, is attributed to the evaporation of H₂O absorbed in the TiO₂ nanorods arrays. The peak at 700-800° C. might be contributed by the phase transition of TiO₂ nanorod from brookite to rutile structure. Clearly, the main weight loss is contributed by the loss of H₂O absorbed on TiO₂. All DSC data exhibited almost the same plot, further proving that the main mass loss was caused by loss of H₂O.

The specific BET surface area of TiO₂ nanorod arrays on honeycomb monolith grown for different times was also investigated, and the results are listed in Table 1.

TABLE 1 Multi-point nitrogen adsorption BET specific surface area of TiO₂ nanorods grown on honeycomb monolith substrate TiO₂- TiO₂- TiO₂- TiO₂- Blank- substrate substrate substrate substrate Sample substrate (1.5 h) (2 h) (4 h) (24 h) Surface Area 24.123 23.264 32.733 68.251 82.934 (m²/g, with substrate) Surface Area 0 66.584 358.750 527.810 641.360 (m²/g, without substrate)* *BET Surface Area (estimate without substrate) were calculated according to the following rule: a) Assuming TiCl₃ was completely converted into TiO₂ grown on the substrate. b) For growth time shorter then 4 h, TiO₂ nanorods array not completely covered the substrate from SEM, so S = (S_(TiO2-substrate) − S_(Blank-substrate)) * M_(TiO2-substrate)/m_(TiO2). c) For growth time longer than 4 h, TiO₂ nanorods arrays completely covered the substrate from SEM, so S = S_(TiO2-substrate) * m_(TiO2-substrate)/m_(TiO2)

As shown in Table 1, TiO₂ nanostructures grown for 1.5 h have a relatively low BET surface area. That may be caused by the lower coverage of TiO₂ nanorod clusters on the monolith substrate. BET surface area of TiO₂ nanostructures grown for 2 h is larger (see Table 1) than that of the sample synthesized for 1.5 h. This could be attributed to the higher aspect ratio of single nanorods grown out of the nanoclusters and higher TiO₂ coverage on the monolith. Although having similar nanorod array structure, the BET surface area of TiO₂ nanostructures grown for 24 h is larger than the surface area of the sample grown for 4 h, which is due to the larger aspect ratio of TiO₂ nanorods with increasing growth time. As shown in Table 1, the specific surface area of honeycomb monolith substrate increases after the deposition of TiO₂ nanorods. With the increasing growth time, the surface area increases drastically to 3-4 times that of bare substrate. It is worth noting from the data shown in Table 1 that the grown TiO₂ nanorod arrays have a specific surface area as high as about 700 m²/g, about 3 times higher than the normal hydrolysis derived anatase TiO₂ nanoparticles.

Single crystalline brookite TiO₂ nanorod arrays were grown uniformly on the inner walls of 3D honeycomb monolith by a hydrothermal approach. The evolution of TiO₂ nanorod clusters to TiO₂ nanorod arrays were observed while changing the hydrothermal treatment time. Slow oxidation of Ti(III) to Ti(IV) results in the maintenance of a low degree of supersaturation, which promotes the heterogeneous nucleation of brookite TiO₂ on the wall surface of honeycomb monolith. In addition, the presence of excess Cl⁻ helps the formation of brookite TiO₂ and maintains the brookite crystal structure. The synthesized TiO₂ nanorods-based hybrid nanocatalyst support have high surface area and good thermal stability, which can be applied in various energy and environmental applications such as emission control, industrial filtering, purification, and separation. An example of CO oxidation results with a TiO₂ nanorod-based 3D nanocatalyst is shown in FIG. 7. Other applications of nanorod structures are listed in Table 2.

FIGS. 8A-F show examples of cordierite and stainless steel substrates (FIG. 8A), and higher magnification SEM images of cordierite substrate channels (FIG. 8B) and stainless steel substrate channels (FIG. 8C), as well as top view images of TiO₂, ZnO, and Al₂O₃ nanorod arrays (FIGS. 8D, 8E, and 8F, respectively) grown on cordierite or stainless steel substrates.

TABLE 2 Nanorod Array Materials Applications Materials Applications Remarks ZnO Catalysts ZnO/Cu industry methanol synthesis and water-gas shift catalysts; Photocatalysts. Desulfur and De-As Typical industry syngas desulfur absorber (H₂S); filter As-ion removal in water treatment; ZnO—NiO petroleum industry oil desulfur catalysts; Cigarette filters. Electrodes Transparent electron transport layer in solar cell and electronic devices. Personal care Antibacterial lotion; an ingredient in sunscreen; Sensors Gas sensors; biological sensors. Actuators/MEMS Piezoelectric devices TiO₂ Catalysts Catalytic supports in automobile exhaust emission system; Photocatalysts for H₂ generation, organic pollute degradation. Sensors Gas sensors and humidity sensors Personal care an ingredient in sunscreen; Electrode Main components in dye-sensitized solar cell. CeO₂ Catalysts Automobile emission control: CeO₂—ZrO₂ for NO_(x) storage/Reduction Electrode materials for Environmental monitoring: O₂, NO₂, SO₂, etc. sensors Oxygen conductors Solid oxide fuel cells: CeO₂—Y₂O₃ Personal care Ultraviolet blocking component in cosmetics Abrasives Chemical-mechanical planarization: single-crystal ceria nanospheres High temperature CeO₂ as an insulating layer superconductive materials Al₂O₃ Catalysts Supports for catalytically active metals Gate oxides Dielectric materials for electronics Co₃O₄ Electrode Catalysts Li-ion Battery electrodes, industrial catalysts and supports LSMO, catalysts Gas phase catalysis LSCO Electrical and surface conductivity or ionic conductivity change electrochemical (through solid electrolyte) upon exposure to various Sensors gases Fuel cell electrodes High/medium temperature solid oxide fuel cell Magnetic sensing and Colossal magetoresistance effect information storage Pt, Au, Pd Catalysts Main catalytic active site in all kinds of catalysts. Sensors Electrodes and sensitivity/selectivity enhancement BaO, K₂O, catalysts NOx storage materials. Li₂O

In still another embodiment, a method of making metal oxide nanowire arrays includes electrodepositing metal oxide onto a substrate having a surface and multiple channels from an electrolytic solution of metal-chloride or -nitrate and potassium chloride, the metal oxide nanowire arrays being coupled to the substrate surface via an interface layer. The concentration of metal-chloride or -nitrate and potassium chloride can each independently be in a range of between about 0.001 M and about 1.0 M. The electrodeposition can be conducted at a temperature in a range of between about 25° C. and about 90° C., such as about 70° C. The metal oxide can be selected from the group consisting of: cerium oxide, zinc oxide, titania, zirconia, and tin oxide. Electrodeposition parameters for making exemplary metal oxide nanowire arrays are listed in Table 3.

TABLE 3 Electrodeposition parameters of metal oxide nanorod/wire arrays. Metal Precursor Additive Electrodeposition Electrodeposition Electrodeposition Oxide Precursor Concentration (M) Additive Concentration (M) method Temperature (° C.) Time (h) CeO₂ Ce(NO₃)₃ 0.001-1 KCl or 0.01-1 Galvanostatic or 25-90 0.25-5 or CeCl₃ NH₄Cl Potentiostatic ZnO Zn(NO₃)₂ or or ZnCl₂ NH₄Ac TiO₂ TiCl₃ SnO₂ SnCl₂ or Sn(NO₃)₃ ZrO₂ ZrCl₃ or Zr(NO₃)₃

EXEMPLIFICATION

Materials and Methods

Preparation of Single Crystalline Brookite TiO₂ Nanorod Arrays on Ceramic Monoliths.

Nanostructured brookite TiO₂ was synthesized by hydrothermal treatment of aqueous titanium trichloride (TiCl₃) solutions with saturated sodium chloride (NaCl). The dilute solution of TiCl₃ (0.15 M) was supplied by Spectrum Chemical MFG. Corp. Typically, 20 ml of the precursor solution, including 2 ml of TiCl₃ solution and 18 ml of deionized (DI) water saturated with NaCl, was placed in a Teflon-lined autoclave. Honeycomb cordierite monoliths were used as substrates and were put in the solutions. The solutions were then heated at 180° C. for 1-24 h. Then, the autoclave was cooled to room temperature. The honeycomb monoliths were taken out, rinsed thoroughly with deionized water and allowed to dry on a hot plate at 80° C.

Scanning Electron Microscopy, Transmission Electron Microscopy and X-Ray Diffraction.

The morphologies of the synthesized TiO₂ nanostructures were characterized by field-emission scanning electron microscopy (FESEM; JEOL 6335F Field Emission SEM) equipped with a Thermo Noran EDS detector, and by high-resolution transmission electron microscopy (HRTEM; JEOL 2010 FasTEM). The crystal structures were examined by X-ray diffraction (XRD) analysis with a Rigaku MiniFlex II Desktop X-ray Diffractometer.

BET Surface Area and Thermal Stability Measurements.

The BET surface area was characterized using Quantachrome Corporation NoVA 1000 Gas Sorption Analyzer. The thermal stability of samples was investigated using TA instruments SDT Q600.

3-Step Process Example: ZnO/LSCO/BaO Composite Nanowire Arrays on Cordierite or Other Substrates.

Dip-Coating of ZnAc₂+350° C. Annealing, 5-10 Cycles; Sonication Helps Uniformity.

The monolith substrate was first rinsed by deionized water and ethanol in ultrasonic bath and then coated (via dip-and-dry) with 1-100 mM, such as about 10 mM, zinc acetate (ZnAc₂) in ethanol solution. After the sample was dipped into the zinc acetate ethanol solution, the solution with the sample was then put into an ultrasonic bath for 0.5-10 minutes, such as about 5 minutes. The substrate was then heated and dried on hotplate at 120-200° C. for 1-10 minutes. The previous steps were repeated for 3-10 times to ensure enough seed nanoparticles stick onto the substrate surface. The dip-coated substrate was then annealed at 300-400° C., such as about 350° C., for 1-10 hours, such as about 2 hours.

Then, after the film position, the ZnO seed layer coated substrate was put into a covered glass bottle containing an aqueous solution of a precursor made of equal molar amounts of Zn(NO₃)₂ and hexamethylenetetramine (HMT) of a concentration in a range of between about 5 mM and about 100 mM, such as about 10 mM, dissolved in 100-1000 mL of deionized water. The pre-annealed dip-coated substrate was then grown in the prepared precursor at different temperature, in a range of between about 60° C. and about 95° C., such as about 80° C. After about 1-10 hours, such as about 5 hours, the substrate grown with ZnO nanorods was taken out and cleaned in an ultrasonic bath with water or ethanol for 1-20 minutes, such as about 5 minutes. Finally, the sample was dried at 70-95° C., such as about 80° C., in air for further processing. The length of ZnO nanorods was mainly controlled by its growth time. Other precursor solutions, temperatures, and growth duration times for hydrothermal synthesis of the respective nanorod arrays are listed in Table 4.

TABLE 4 Hydrothermal synthesis of various metal oxide nanorod arrays Cation-contained Duration time Nanowires precursors Temperature (° C.) (hours) SnO₂ SnCl₂ or Sn(NO₃)₂ 160-240 6-72 ZrO₂ ZrCl₃ or Zr(NO₃)₃ 180-240 6-72 Al₂O₃ AlCl₃ or Al(NO₃)₃ 160-240 12-48  CeO₂ CeCl₃ or Ce(NO₃)₃ 120-200 12-36  Ga₂O₃ Ga(NO₃)₃ 120-240 6-48 Co₃O₄ Co(NO₃)₂  50-180 1-24 Dip-Coating of LSCO+BaO Precursor Solution Followed by 700-800° C. Annealing, 5-10 Cycles; Stirring Helps Uniformity.

For the deposition of LSCO and BaO thin films onto the ZnO nanorod arrays, a LSCO+BaO precursor solution was prepared by dissolving lanthanum nitrate hexahydrate (La(NO₃).6H₂O, 1-20 mM, such as about 5 mM), strontium nitrate (Sr(NO₃)₂, 0.1-10 mM, such as 0.5 mM), cobalt nitrate (Co(NO₃)₃, 1-20 mM, such as about 5 mM), and barium nitrate (Ba(NO₃)₂, 1-20 mM, such as about 5 mM) in ethoxyethanol (100 mL) under vigorous stirring and sonication at about 50-70° C., such as about 60° C. Suitable amounts of polyvinylpyrrolidone (Mw 55000, 0.1-2 gram, such as about 0.5 mM) and diethanolamine (0.5-10 mL) were added to make the suspension solution transparent, which was followed by an aging process of 2-4 days. Then, the nanorod coated substrate was dipped into the precursor solution, the solution with the substrate was sonicated for 0.5-5 minutes, such as about 1 minute, and then the substrate was dried at 120-200° C. for 1-10 minutes, such as about 5 minutes, followed by annealing at 250-400° C. for 0.2-1.0 hours, such as about 0.5 hours, and 600-800° C., such as about 700° C., for 0.5-2 hours to ensure good crystallinity and adhesion of the coating. Depending on the thickness requirement, 1-10 cycles of dipping and drying were needed before the annealing. BaO can be loaded after LSCO deposition as well, which become the fourth step.

For the loading of K₂O, the precursor used is KNO₃, with concentration of 1-20 mM depending on the loading amount. The experimental procedure is similar to the LSCO and BaO loading procedure described above.

Synthesis of CeO₂ Nanowires on Stainless Steel Substrates.

Electrochemical Deposition

Electrochemical preparation was carried out in a conventional three-electrode cell via galvanostatic electrodeposition. The working electrodes were stainless steel substrates. A platinum wire was used as the auxiliary electrode. An Ag/AgCl electrode was used as the reference electrode. The electrolytic solution contained CeCl₃ (0.001-0.1 M). All reagents used were analytical grade and were used directly without any purification. The reaction temperature was kept at 70° C. SEM images of CeO₂ nanorods on a stainless steel substrate are shown in FIGS. 9(a)-(d).

Loading of Precious Metal (Pt, Au, Pd, Ru, Etc)—Pt Deposition Used as Example

Method 1: Colloidal Deposition Method

The preparation of the Pt/MO_(x) nanorods catalyst was performed by the colloidal deposition method. Firstly, unprotected Pt nanoparticle colloid was synthesized by an ethylene glycol (EG) method known in the art. Typically, all operations were carried out under inert atmosphere (Ar or N₂). In a typical preparation, a glycol solution of NaOH was added into a glycol solution of H₂PtCl₆.6H₂O with stirring to obtain a transparent yellow platinum hydroxide or oxide colloidal solution, which was then heated at 160° C. for 3 h, with an Ar flow passing through the reaction system to take away water and organic byproducts. A transparent dark-brown homogeneous colloidal solution of the Pt metal nanocluster was obtained without any precipitate. For deposition, as prepared Pt colloid was diluted by adding 19 times EG solvent. Secondly, cordierite monolith substrate with TiO₂ nanorods grown on it was placed in the Pt colloid solution for 20 minutes with magnetic stirring. Then the sample was taken out and dried at 100° C. in air. After repeating the procedure 3 times, Pt/TiO₂ nanorods catalysts grown directly on cordierite monolith substrate were obtained.

Method 2: Impregnation Method.

A monolith substrate with MO_(x) nanorods was place in Pt(OH)_(x) solution for 20 mins with magnetic stirring. Then the sample was taken out and dried at 500° C. in air. After repeating the procedure for 3 times, Pt/MO_(x) nanorods catalysts grown directly on cordierite monolith substrate were obtained.

Method 3: In-Situ Reduction Method.

The monolith substrate with MO_(x) nanorods and PVP solution as a stabilizer were added into H₂PtCl₆ solution. A reductant solution (NaBH₄) was injected into the H₂PtCl₆ solution. Meanwhile, hydrogen gas was also injected into the rector to vigorously stir the solution and to make the reaction homogeneous. The synthesis process was stopped after complete consumption of the NaBH₄ solution. Then, the product was filtered and washed until Cl⁻ was completely removed according to a test with AgNO₃, and the final products were dried in an oven to obtain the desired Pt/MO_(x) nanorods catalysts.

Heterogeneous Nanowire Array Growth Procedures.

Depending on the number of compounds or layers involved in the multi-component heterogeneous nanowires, the growth procedure of heterogeneous nanowires will involve the corresponding number of growth processes. Taking ZnO/LaCoO₃/Pt nanowire as example, three steps will be involved in the fabrication process:

Step 1: hydrothermal synthesis of ZnO nanowires;

Step 2: physical vapor deposition (pulsed laser deposition (PLD) or sputtering) or sol-gel deposition of LaCoO₃ nanoparticles onto ZnO nanowires;

Step 3: impregnation or colloidal deposition method is used to growth Pt nanoparticles onto the formed ZnO nanowires.

Similarly, TiO₂/LaCoO₃/Pt nanowire arrays on monolith substrates can be fabricated using hydrothermal, sol-gel (or PLD, sputtering) and impregnation methods.

Further, TiO₂/LaCoO₃/BaO(K₂O)/Pt nanowire arrays on monolith can be fabricated simply using a combination of hydrothermal, sol-gel (PLD) and impregnation.

Another Example of TiO₂ Nanorods Arrays on 3-D Cordierite Substrate.

Recent studies on sulfur durability of NO_(x) storage/reduction (NSR) catalysts have reported that TiO₂ in support materials provides high tolerance against sulfur poisoning due to the high acidity/low basicity of TiO₂. Since TiO₂ has high thermal stability and high surface area, it is an ideal matrix for 3-D nanostructured NSR catalysts. TiO₂ nanorod arrays were grown on 3-D cordierite substrate by a hydrothermal method. As-synthesized TiO₂ nanorods arrays show a highly uniform, aligned and densely packed array of nanorods on the inside wall of ceramic honeycomb substrate (FIG. 10A). The morphology of TiO₂ nanorods was quite uniform with diameter in a range from 50 nm-130 nm and length in a range from 500 nm-1 μm. The EDS analysis (inset of FIG. 10A) confirmed the synthesis of TiO₂. From the cross-sectional view of the same sample (FIG. 10B), it can seen that TiO₂ nanorods grow almost perpendicularly out of the cordierite channel wall with a length more than 500 nm. FIG. 10B also shows the substrate 10, the interface layer 20 bonded to the surface of the substrate 10, and the vertically aligned TiO₂ nanorod array 30 coupled to the substrate 10 via the interface layer 20. Transparent electron microscope (TEM) gives more detailed structural information about the TiO₂ nanorods. FIG. 10C shows the TEM image of a typical single TiO₂ nanorod, which is very straight and uniform in the whole body. The inset of FIG. 10C is a selective area diffraction pattern from as-shown single nanorod, showing that the TiO₂ nanorod grows from a direction. High Resolution TEM image (FIG. 10D) demonstrates that the nanorod is completely crystallized with clear lattice fringes parallel to the wall. An interplanar spacing of 0.34 nm is clearly imaged corresponding to the (11-1) plane perpendicular to the nanorod axis. The TEM images imply that the nanorod grows along the (11-1) crystal plane with a preferred [301] direction. The selected area electron diffraction (SAED) pattern (inset of FIG. 10C) confirms the single-crystal structure of brookite-TiO₂ nanorods with [11-1] being the zone axis. X-ray diffraction (XRD) patterns (FIG. 10E) also confirmed the brookite structure of as-synthesized TiO₂ nanorod arrays. It can be found that brookite-TiO₂ and rutile-TiO₂ are present together, with brookite-TiO₂ being the main phase. A high concentration of chloride ions in the hydrothermal medium contributes to the formation of brookite-TiO₂ and also avoids the crystal transformation from brookite to rutile.

Examples of Metal Oxide (CeO₂, ZnO, SnO₂, Al₂O₃, ZrO₂, and TiO₂) Core-Shell Composite Nanorod Arrays on 3D and 2D Substrates.

Using ZnO—CeO₂ as a Representative Example.

Cerium dioxide (CeO₂) is one of the most important rare-earth materials, which has diverse applications as support components for three-way catalysts, and catalysts for selective hydrogenation. One-dimensional (1D) CeO₂ nanostructures such as nanorods, nanowires and nanotubes have attracted extensive attention due to improved redox properties and transport properties as compared to CeO₂ nanoparticles. The assembly of 1D CeO₂ nanostructures into ordered three-dimensional (3D) nanostructures is very necessary for the development of high performance nanodevices. By use of sputtering methods and wet chemical methods, large scale ZnO/CeO₂ nanorod arrays have been successfully fabricated with well controlled dimensionality, crystal structures, and device structures. Firstly, ZnO nanorod films have been fabricated on the sintered 2-D Si(100) substrates and 3-D cordierite or stainless steel substrate by a hydrothermal method. Then, CeO₂ composite films have been deposited on ZnO nanorod arrays by magnetic sputtering process. The morphologies and crystal structures of the ZnO/CeO₂ composite nanorod arrays were characterized with field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM). FIGS. 11A and 11B show the top-view and cross-sectional view SEM images of large area ZnO/CeO₂ nanorod arrays, respectively. From the two images, one can find that the ZnO/CeO₂ nanorods have diameters in a range from 30 nm to 70 nm and length of 1 μm. Compared with ZnO nanorods, the rough surface of ZnO/CeO₂ is an indication of CeO₂ nanofilm grown on the surface of the ZnO nanorod. More structural information about ZnO/CeO₂ nanorods was obtained by using TEM. FIG. 11C shows the bright-field TEM image of a typical ZnO/CeO₂ core-shell nanorod, while FIG. 11D depicts the corresponding dark-field image of the same ZnO/CeO₂ nanorod. From the two images, one can clearly see that there is a uniform polycrystalline CeO₂ nanofilm of 10-50 nm thickness that was coated on the about 100 nm wide ZnO nanorod. FIG. 11E shows the corresponding selected area electron diffraction pattern. From EDS results (FIG. 11F), one can see element information of Zn, O and Ce, which suggest the successful fabrication of ZnO/CeO₂ nanorods. These unique nanostructured arrays could be useful as catalysts on organic dye elimination and hazard gases control.

Examples of Metal Oxide (CeO₂, ZnO, SnO₂, Al₂O₃, ZrO₂ and TiO₂) Heterogeneous Nanorod Arrays.

FIGS. 12A-12D show SEM images of Al₂O₃—TiO₂ heterogeneous nanorod arrays.

Examples of Metal Oxide (Containing CeO₂, ZnO, SnO₂ and TiO₂, Al₂O₃, ZrO₂)/ABO₃ Perovskite (Such as La-Based Transition Metal Oxides, LaMO₃, M=Co, Cr, Fe, Mn, and Ni) Mesoporous Nanocomposite Structure Arrays on 3D and 2D Substrates

Using TiO₂/(La, Sr)MnO₃ (LSMO) as a Representative Example.

TiO₂ nanorod arrays were grown on 2-D glass and silicon, 3-D cordierite and stainless steel substrates. By using an RF magnetron sputtering method, TiO₂/LSMO core-shell nanorod arrays were fabricated on either 2-D or 3-D substrates. First, TiO₂ nanorod arrays were prepared on the substrate by the hydrothermal method described above. After that, TiO₂/LSMO nanorod arrays were prepared from TiO₂ nanorod arrays through RF magnetron sputtering LSMO nanofilm on the surface of TiO₂ nanorod arrays. FIG. 13A shows the top-view of TiO₂/LSMO nanorod arrays on higher magnification. FIG. 13B is the cross-view image of TiO₂/LSMO nanorod arrays. The TiO₂/LSMO nanorods have diameters in a range from 30 nm to 70 nm and length of 1 μm. By using TEM, more structure information about TiO₂/LSMO nanorods was obtained. FIG. 13C and FIG. 13D depict the TEM image of TiO₂/LSMO nanorods, from which the core of the TiO₂ nanorod and the shell covered on it can be seen. EDS results (FIG. 13E) suggest the successful fabrication of TiO₂/LSMO nanorods. This kind of structure will have improved performance on oxidizing of NO or CO into NO₂ and CO₂ in exhaust emission catalytic processes.

Examples of Metal Oxide (Containing CeO₂, ZnO, SnO₂ and TiO₂, Al₂O₃, ZrO₂)/Precious Metal Nanoparticles (Pt, Pd, Au, Ru, and Rh) Mesoporous Nanocomposite Structure Arrays.

After growth of TiO₂ nanorods on cordierite honeycomb substrate, Pt nanoparticles were loaded on the TiO₂ nanorods arrays by a simple impregnation method. FIGS. 14A-14D show the transmission electron microscopy (TEM) images of the synthesized TiO₂/Pt nanocomposite. FIG. 14A is the bright field TEM image of a few TiO₂/Pt nanocomposite nanorods, while FIG. 14B exhibits the corresponding dark field TEM image of the nanorods. Pt nanoparticles loaded on the TiO₂ nanorods could be clearly observed with high dispersion from either of the two pictures. Pt nanoparticles in a single nanorod were also investigated in a higher magnification (FIG. 14C). The precipitated Pt nanoparticles were quite uniform, the sizes of which were in a range from 2 nm to 10 nm with average diameter of 3 nm. The high-resolution TEM (FIG. 14D) revealed the highly crystalline features of the support TiO₂ nanorod as well as the Pt particle. The fringes with a lattice spacing of 0.194 nm in the particle can be indexed as (200) plane of face-centered cubic (fcc) Pt. The fringes with lattice spacing of 0.243 nm and 0.218 nm are corresponding to the (101) plane and (111) plane of rutile-TiO₂, respectively.

Examples of Metal Oxide (Containing CeO₂, ZnO, SnO₂ and TiO₂, Al₂O₃, ZrO₂)/ABO₃ Perovskite (Such as La-Based Transition Metal Oxides, LaMO₃, M=Co, Cr, Fe, Mn, and Ni)/Bao, K₂O/Precious Metal Nanoparticles (Pt, Pd, Au, Ru, and Rh) Mesoporous Composite Nanoarray Catalysts.

Those 3-D composite nanomaterial systems effectively improve the performance of commercially used three-way catalysts, diesel oxidation catalysts (DOC), and NO_(x) removal catalysts, such as lean NOx trap (LNT) and selective catalytic reduction (SCR) in automobile and remove harmful gases from stationary energy plants. They can also be used in other areas, such as new energy generation and storage devices and selective separations in chemical and biological industries.

Robust 3-D Configured Metal Oxide Nano-Array Based Monolithic Catalysts with Ultrahigh Materials Usage Efficiency and Catalytic Performance Tunability.

By integrating 3-D monolithic substrates with metal oxide nanostructure arrays (nano-arrays) and Pt nanoparticles using low cost solution-phase methods, a new class of nano-array structured catalysts has been successfully fabricated. These 3-D configured nano-array catalysts have greatly enhanced the device surface area, the surface Pt-nanoparticle dispersion and active site population, and demonstrated strong nanostructure-substrate adhesion and tunable nanostructure shape and size. In the meantime, efficient gas diffusion and catalytic reaction have been enabled by the ordered arrangement of the nano-array assembled nanochannels. As a result, 10-40 times less mass loading was achieved in both Pt and support metal oxides in nano-array catalysts as compared to classical washcoated powder-form counterparts. Meanwhile, these nano-array catalysts consistently demonstrated excellent, robust, and tunable catalytic performance on CO oxidation. The 3-D configured nano-array based monoliths represent a new general strategy to fabricate highly efficient and robust nanostructure based catalytic devices for various sustainable industrial applications.

For decades, three-dimensional (3-D) nano-array structured materials have been broadly introduced to enable improved or unique performance in electronics, optical devices, lithium-ion batteries, solar cells, light-emitting diodes, microfluidics, biomedical devices, self-cleaning surfaces and super adhesives. Meanwhile, nanostructure (such as nanoparticles, nanowires) based catalysts in powder-form have been intensively investigated and are regarded as the potential next generation catalysts. However, the study of array structured 3D nano-architectures with ordered arrangement of macropores and mesopores and with the nano-size effect has been largely neglected as unique structured nanocatalysts for industrial catalytic processes. The main challenges or obstacles are due to the lack of a well-defined 3D platform as well as an effective integration strategy.

In industrial catalytic processes, monolithic catalytic devices, such as catalysts, filters, and reactors, are generally more efficient and cost-effective as compared with powder or pellet ones, as a result of a few outstanding merits including low pressure drop, high geometric surface area, efficient mass-transfer, and relatively low catalyst usage. Consequently, they have been important devices as either dedicated or alternative solutions in catalytic combustion, chemical production, separation and purification, and environmental pollutant treatment such as water purification, vehicle and industrial exhaust emissions treatment. However, three main issues remain challenging for the research and development of powder washcoated monolithic catalytic devices: (i) the inevitable use of precious metals (Pt, Rh, and Pd) in some applications such as exhaust treatment makes them expensive and in limited supply; (ii) empirical washcoated powder-form catalysts lack well-defined structural and geometrical configurations, severely compromising the catalytic performance and materials utilization efficiency of these catalysts; (iii) current understanding of the relationship between practical industrial catalysts' performance and the origin of catalytic activity, i.e., atomic and nanoscopic surfaces and interfaces in the comprised individual micro- and nanostructures, is quite limited.

Herein, through a general, low-cost, green and robust hydrothermal process, the inventors directly grew crystalline nano-arrays of support metal oxides such as ZnO, TiO₂, CeO₂, and Co₃O₄ onto commercial cordierite and stainless steel monolithic substrates (FIG. 15a ), which not only could serve as an idealized 3-D platform for investigation and application of array structured nanocatalysts but also show the potential to effectively upgrade the state-of-art powder washcoated monolithic catalysts by significantly mitigating the above mentioned problems. It may, for the first time, provide a widely applicable and easily extendable catalytic device platform based on scalable and tunable 3-D nanostructure assembly. The nano-array structured catalysts have greatly enhanced the devices' surface area, the array-surface Pt-nanoparticle dispersion and active site population, and demonstrated strong nanostructure-substrate adhesion and tunable nanostructure shape and size. In the meantime, efficient gas diffusion and catalytic reaction have been enabled due to the ordered arrangement of the macroporous channels in the array structures. As a result, with 10-40 times less mass loading in both Pt and support metal oxides as compared to classical washcoated powder-form ones, these nano-array monolithic catalysts demonstrated consistently excellent, robust, and tunable catalytic performance on CO oxidation, which was conducted as a probe reaction based on its great importance in both practical application (e.g., exhaust pollutant treatment, fuel cell) and fundamental research. See Qiao, B.; Wang, A.; Yang, X.; Allard, L.; Jiang, Z.; Cui, Y.; Liu, J.; Li, J.; Zhang, T. Nat. Chem. 2011, 3, 634-641; Liu, L.; Zhou, F.; Wang, L.; Qi, X.; Shi, F.; Deng, Y. J. Catal. 2010, 274, 1-10.

To prepare the nano-array assembled monolithic catalysts, a two-step procedure was employed. In the first step, a low temperature hydrothermal method was used for the growth of various metal oxide (ZnO, TiO₂, Co₃O₄) nanorod arrays directly onto monolithic substrates by using the respective precursors containing Zn, Ti, and Co cations. CeO₂ nanotube arrays were grown on monoliths using ZnO nanorod arrays as template with additional hydrothermal deposition of CeO₂ nanoshells in one pot, followed by template removal by wet chemical etching. The Pt nanoparticles were loaded using dip coating of colloid Pt nanoparticles made from thermal decomposition of Pt-contained hydrochloride salts. Typically, the required amount of as-prepared Pt colloid (Pt/cordierite=0.1 wt %) was diluted by adding acetone as solvent to the volume which can totally cover cordierite monolith substrates with nanostructure array. The immersed substrates were placed into the diluted Pt colloid solution with magnetic stirring until the solution is dry. Finally, the sample was dried at 80° C. in air, and Pt/MO nano-array monolithic catalysts were ready to use. The overall weight percentage of Pt on the nano-array was confirmed by statistical TEM EDXS analysis and inductively coupled plasma-atomic emission spectroscopy (ICP-AES), which was in a good agreement with Pt amount in the initial supply, suggesting that most of Pt nanoparticles had been loaded on the metal oxide nano-arrays. (see below for details).

FIG. 15b shows the photographic image of four types of nano-array based monolithic catalysts including ZnO, TiO₂, CeO₂, and Co₃O₄. Scanning electron microscope (SEM) top-view images of ZnO nanowire array rooted cordierite monolith channels are shown in FIG. 15c at low magnification. The as-synthesized Co₃O₄ nanowires (about 10 μm long) in FIGS. 15d-15e exhibited a high uniformity as well as well-aligned and densely packed array characteristics. The closer view electron micrographs in FIGS. 15f-15m revealed the well-defined structural and morphological characteristics of the grown metal oxide nano-arrays on monolith substrates. For instance, FIGS. 15f (SEM) and 15 j (TEM) identified the ZnO nanowire arrays with good uniformity and coverage, single crystallinity, a diameter of about 60 nm, and [0001] growth direction. Table 5 summarizes the physical, chemical, and structural characteristics of the as-grown metal oxide nano-arrays (described below).

TABLE 5 Physical, chemical, and structure characteristics of grown nano-arrays on bare monolith substrates. Array Diameter (nm)/ Wall thickness or Structure/ Growth d-spacing (nm)/ Images Materials morphology Length (μm) pore size (nm) Crystallinity direction crystal plane (FIG. 15) ZnO nanorod/ 50-150/1-5 — Wurtzite/ [0001]  0.52/{0001} c, f, j nanowire Single  0.28/{01-10} crystal TiO₂ nanorod  50-130/0.5-1 — Brookite/  [001] 0.35/{120} g, k Single 0.51/{001} crystal CeO₂ nanotube 70-200/1-5 10-20 Fluorite/ — 0.31/{111} h, l Polycrystal Co₃O₄ porous 100-200/1-12 20 (10-50) Spinel/ — 0.46/{111} d, e, i, m nanowire Polycrystal 0.28/{220}

Thermal and mechanical stability of monolithic catalysts is important for various catalytic processes at elevated temperature, such as catalytic combustion, steam reforming, and automobile exhaust after treatment. In vehicular applications, the monoliths are also subjected to excessive mechanical vibrations. Therefore, the inventors conducted a series of time dependent studies on thermo-gravimetry, surface area changes, structure and morphology evolution, and mechano-gravimetry. It is noted that the thermal and mechanical stability of two-step template-induced CeO₂ nanotube array (see below) was not as good as those of the directly grown nanowire array (ZnO, TiO₂, and Co₃O₄) based monolithic catalysts (as illustrated in FIG. 16), which is due to the polycrystalline nature of the nanotubes and their non-uniform tube wall thickness along the nanotube length. For the nano-array monoliths based on ZnO, TiO₂, and Co₃O₄, less than 1% mass change was observed after a 100-hour 800° C. isothermal annealing process. The corresponding XRD spectra revealed no phase segregation for ZnO, TiO₂, and Co₃O₄ nano-array monoliths. From inset SEM images, the ZnO and TiO₂ nano-array structures remained intact despite the gradually smoothened edges of the nanorods. In Co₃O₄, the as-prepared porous nanowires with relatively smooth surfaces turned into zigzag periodic and rough nanowires after 24 hours of annealing at 800° C. (FIG. 16c ). However, the morphology of Co₃O₄ nanowires was retained after 24 hours. The pore size distribution changed only slightly with an average 20 nm pore size maintained throughout the 800° C. isothermal process (FIG. 29), but the pore volume dropped by about 46% after the first 24 hours, indicating a decrease of pore population. The measured BET surface area of a TiO₂ nanorod array monolith decreased by about 43%, from 57 m²/g to 33 m²/g after 24-hr annealing at 800° C. After that, surface area stayed constant throughout the remaining 76-hr isothermal process. Similar to TiO₂ nanorods, the surface area of a Co₃O₄ nanowire array monolith decreased from 8 m²/g to 4 m²/g after 24-hr annealing at 800° C. However, the surface area of a ZnO nanorod array monolith remained constant in the 100-hr annealing at 800° C. Compared with the significant surface area decrease in conventional powder-form catalysts (50% decrease in ZnO; >80% decrease in TiO₂ and Co₃O₄) under high temperature, ZnO, TiO₂ and Co₃O₄ nano-array monoliths demonstrated much higher thermal stability in both short-term and long-term scales (5%, 43%, and 56% surface area decrease for ZnO, TiO₂, and Co₃O₄, respectively). As-prepared nano-array structures also exhibited good thermal stability in reductive atmosphere (see below and FIG. 30). The well-separated but densely-packed nano-arrays effectively defined and utilized spaces between individual nanostructures with high surface area, therefore suppressing the sintering induced densification and grain growth.

The nano-array adhesion and structure stability was examined by applied high velocity air flow (static 50 L/min or pulsatile flow) for 10 days while monitoring morphological variation and weight loss. The results summarized in the right panel of FIG. 16 suggested that ZnO, TiO₂, and Co₃O₄ nano-array monoliths exhibited excellent mechanical robustness and the nano-array morphology was retained with only minor change throughout. The weight losses of the above three nano-array monolithic catalysts were less than 1% individually.

The rational design and assembly of metal-metal oxide interface through shape and size control is emerging as one of the most effective routes to tailor the catalysts' surface atomic and electronic structures and thus their catalytic activity and selectivity. See Xie, X.; Li, Y.; Liu, Z.-Q.; Haruta, M.; Shen, W. Nature 2009, 458, 746-749. FIGS. 17a-17d display the individual ZnO, TiO₂, CeO₂, and Co₃O₄ nanostructures loaded with well-dispersed 2 nm Pt nanoparticles on their well-defined crystal surfaces with the inset lattice images clearly identifying the exposed Pt(111) atomic planes. The light-off temperatures (where 50% of CO conversion is achieved) for CO oxidation of Pt-loaded CeO₂, ZnO, Co₃O₄, and TiO₂ nano-array monolithic catalysts are 193° C., 260° C., 195° C., and 258° C., respectively and 100% CO conversion was achieved below 300° C. (FIG. 17e ). The better CO oxidation performance in Pt/CeO₂ nanotube and Pt/Co₃O₄ porous nanowire arrays is due to the promotion effect of Co₃O₄ and CeO₂ nanostructure support to Pt nanoparticle. See Törncrona, A.; Skoglundh, M.; Thormählen, P.; Fridell, E.; Jobson, E. Appl. Catal., B 1997, 14, 131-145. FIG. 17f shows that the ZnO, Co₃O₄, and TiO₂ nano-array supported Pt monolithic catalysts all displayed a very stable 100% CO conversion to CO₂ at T₁₀₀ throughout the 24 hours. In Pt/CeO₂ nano-array catalysts, 100% CO conversion was only sustained during the initial 6 hours. The catalytic activity degradation is caused by this catalyst's relatively lower thermal and mechanical stability (see below). The loaded Pt nanoparticles on the nano-array monolithic catalysts show little change in size with well-retained nano-array structure for both ZnO and TiO₂ after the catalytic performance and stability tests (FIG. 34), demonstrating their excellent thermal stability.

Compared with the open literature reported catalysts with comparable catalytic performance, the materials utilization efficiency of these nano-array monolithic catalysts (FIG. 17g ) is about 10-40 times higher with extremely small loadings of precious metal and metal oxide nano-arrays used (see below and Tables 6, 7 and 8 below). See also Kim, C. H.; Qi, G.; Dahlberg, K.; Li, W. Science 2010, 327, 1624-1627; Boger, T.; Heibel, A. K.; Sorensen, C. M. Ind. Eng. Chem. Res. 2004, 43, 4602-4611; Ji, Y.; Easterling, V.; Graham, U; Fisk, C.; Crocker, M.; Choi, J.-S. Appl. Catal. B 2011, 103, 413-427; Carlsson, P.-A.; Skoglundh, M.; Thormählen, P.; Andersson, B. Top. Catal. 2004, 30-31, 375-381; Roberts, G. W.; Chin, P.; Sun, X.; Spivey, J. J. Appl. Catal. B 2003, 46, 601 611.

TABLE 6 Catalysts usage and 50% conversion temperature of some typical Pt monolithic catalysts reported in literature. Space CO Mass (metal Mass Flow rate velocity percentage Catalyst oxide, mg) (Pt, mg) (ml/min) (h⁻¹) (vol. %) T₅₀(° C.) Reference Pt/Cordierite N/A 0.25 50 N/A 3% CO in air 223 a Pt/Cordierite N/A 6 50 N/A 3% CO in air 195 a Pt/γ-Al₂O₃ 72 14 500 30,000 1% CO in air 180 b Pt/γ-Al₂O₃ 260 0.52 2000 39,000 0.1% CO, 159-188 c 10% O₂/N₂ a: P. Sonstrom, B. Halbach, S. Tambou Djakpou, B. Ritz, K. Ahrenstorf, G. Grathwohl, H. Weller, M. Baumer, Catal. Sci. Technol. 2011, 1, 830. b: R. M. Ferrizz, J. N. Stuecker, J. Cesarano, J. E. Miller, Ind. Eng. Chem. Res. 2005, 44, 302. c: K. Arnby, J. Assiks, P.-A. Carlsson, A. Palmqvist, M. Skoglundh, J. Catal. 2005, 233, 176.

TABLE 7 Catalysts usage and 100% conversion temperature of some typical supported Pt powder catalysts reported in literature. Space CO Mass (metal Mass Flow rate velocity percentage Catalyst oxide, mg) (Pt, mg) (ml/min) (h⁻¹) (vol.) T₁₀₀(° C.) Reference Pt/TiO₂ 77 0.77 154 120,000 1% CO in air 210 d ml/g · h Pt/ZnO—Al₂O₃ ZnO: 3.76 2.08 50 N/A 2% CO/He; 225 e Al₂O₃: 95 air Pt/ZnO 49.5 0.5 100 96,000 0.5% CO; 10% 160 f O₂/Ar Pt/CeO₂ 9 3.5 50 N/A 1% CO; 20% 223 g O₂ in He (mixture) d: O. S. Alexeev, S. Y. Chin, M. H. Engelhard, L. Ortiz-Soto, M. D. Amiridis, J. Phys. Chem. B 2005, 109, 23430. e: K.-J. Kim, H.-S. Seo, H.-G. Ahn, Res. Chem. Intermed. 2011, 37, 1165. f: S. Li, G. Liu, H. Lian, M. Jia, G. Zhao, D. Jiang, W. Zhang, Catal. Commun. 2008, 9, 1045. g: H.-P. Zhou, H.-S. Wu, J. Shen, A.-X. Yin, L.-D. Sun, C.-H. Yan, J. Am. Chem. Soc. 2010, 132, 4998.

TABLE 8 Comparison of catalysts usage between nano-array monolithic catalysts and typical washcoating monolithic catalysts. Metal Precious oxide metal Catalysts layer Catalyst (g/L) (g/L) thickness (μm) reference Nano-array 6.5-88   0.49-0.88 1-10 This work monolithic catalysts Washcoat 98-260  1.77-11.76 15-100 h monolithic catalysts (Al₂O₃ based) h: Kim, C. H.; Qi, G.; Dahlberg, K.; Li, W. Science 2010, 327, 1624-1627; Boger, T.; Heibel, A. K.; Sorensen, C. M. Ind. Eng. Chem. Res. 2004, 43, 4602-4611; Ji, Y.; Easterling, V.; Graham, U.; Fisk, C.; Crocker, M.; Choi, J.-S. Appl. Catal. B 2011, 103, 413-427; Carlsson, P.-A.; Skoglundh, M.; Thormahlen, P.; Andersson, B. Top. Catal. 2004, 30-31, 375-381; Roberts, G. W.; Chin, P.; Sun, X.; Spivey, J. J. Appl. Catal. B 2003, 46, 601 611

It has been predicted that by introduction of macropores into mesoporous catalysts, 20-40% less usage of catalytic material but 80-180% higher activity than the purely mesoporous catalysts can be achieved. Moreover, the reactivity of optimally arranged pore architecture can be 8-10 times better than the reactivity of random pores. See Wang, G.; Johannessen, E.; Kleijn, C. R.; de Leeuw, S. W.; Coppens, M. O. Chem. Eng. J. 2007, 62, 5110-5116; Prachayawarakorn, S.; Mann, R. Catal. Today 2007, 128, 88-99. The 3-D configured nano-arrays have intrinsically ordered macropores between individual nanostructures, which form ordered macroporous channels. These ordered macroporous channels not only enable an optimum metal loading and dispersion during catalyst preparation, but also better expose the available active sites and promote gas-solid interaction by a much shorter diffusion length (FIG. 17h ). The mesoporous nano structure/Pt nanoparticle interfaces may also enable more effective charge transport and thus enhanced reaction efficiency.

The morphology and metal-metal oxide support interaction dependence of catalytic activity in nanostructured catalysts are important for rational design, synthesis, and application of catalysts. To gain such understanding, however, is a non-trivial task. Here, the inventors use the Pt/ZnO nano-array monolithic catalysts as a model platform to practically investigate the catalytic activity dependence on the nanostructure size, shape (crystal facets), and metal-support interaction. Different ZnO nanostructure coated monolithic samples were fabricated as shown in FIGS. 18a-18d . With similar Pt loading, 5.5 μm ZnO nanowire array monolithic catalysts exhibited better CO oxidation catalytic activity than 1 μm ZnO nanorod arrays, and both of them were better than washcoated ZnO powder monolithic catalysts with similar materials loading (FIG. 18e ). The T₁₀ temperature of 5.5 μm Pt/ZnO nanowire array decreased by about 10° C. (220° C.), and light-off (T₅₀) and conversion (T₁₀₀) temperatures decreased by about 30° C. compared to those of the 1 μm one, which are also about 20° C. lower than those of the washcoated powder catalysts. With the calculated catalytic activities (per gram of Pt) at 230° C. for the 5.5 μm and 1 μm Pt/ZnO nanowire arrays being 4.1 mmol and 1.1 mmol, respectively (Table 11), while the activity of corresponding Pt/ZnO washcoated powder catalysts is 0.56 mmol, i.e., about 2-7 times lower. The activity difference between nano-array catalysts and washcoated catalysts further directly confirmed that the arranged macropore distribution could play an important role to improve the catalytic activity. The CO conversion efficiency enhancement with increasing nanowire length may be due to a higher dispersion of Pt nanoparticles on longer ZnO nanowires, associated with their increased chance for strong ZnO—Pt interaction and more exposed active sites. See Liu, B.; Han, S.; Tanaka, K.; Shioyama, H.; Xu, Q. Bull. Chem. Soc. Jpn. 2009, 82, 1052-1054; Pala, R. G. S.; Metiu, H. J. Phys. Chem. C 2007, 111, 8617-8622. On the other hand, to investigate the shape dependence of catalytic behavior, ZnO nano-arrays made of nanorods and nanoplates with the same mass loading were uniformly grown on monolithic substrates. These catalysts had predominantly {0001} or {01-10} crystal surfaces, with identical mass loading over ZnO nano-arrays, normalized amount, crystallinity and size of Pt nanoparticles. The CO light-off curves in FIG. 18f clearly suggest that the Pt/ZnO nanoplate array had an about 20° C. lower T₁₀₀ (242° C.) than the ZnO nanorod array catalysts. Given the identical experimental parameters control, the catalytic activity difference between the nanoplate and nanorod catalysts is due to the different crystal surface area ratio of {0001} to {10-10} (inset of FIG. 18f ), i.e., the different Pt—ZnO facets interaction effect. The interaction of Pt and Zn²⁺ may lead to localized electron transfer on Zn-terminated (0001) surface, resulting in lower CO desorption temperature responsible for the lower CO oxidation temperature in Pt/ZnO nanoplate as compared to that of Pt/ZnO nanorod. See Roberts, S.; Gorte, R. J. J. Chem. Phys. 1990, 93, 5337-5344; Petrie, W. T.; Vohs, J. M. J. Chem. Phys. 1994, 101, 8098-8107.

In summary, the 3-D metal oxide nano-array based monolithic catalysts represent a new and effective model platform for bridging catalytic nanomaterials science and engineering with practical industrial catalysis. It will help enhance predictive catalysis science through enabling exacting relations between nanomaterials tunable in size, shape, and structure, tailorable metal-support interaction, and multifunctional catalytic performances. Therefore, the 3-D configured nano-array based monoliths provide a general strategy for integration of different functional nano-structure arrays with commercially available monolithic substrates, and may bring up a broad array of nanostructure based monolithic devices with ultra-efficient, robust, and multi-functional performance for various catalytic energy, environmental, and biotechnology applications.

Materials and Methods.

Materials

The chemicals used, including zinc acetate dihydrate (Zn(CH₃COO)₂.2H₂O), zinc nitrate hexahydrate (Zn(NO₃)₂.6H₂O), cobalt chloride hexahydrate (CoCl₂.6H₂O), cerium nitrate (Ce(NO₃)₃), sodium chloride (NaCl), lanthanum nitrate hexahydrate (La(NO₃)₃.6H₂O), strontium nitrate (Sr(NO₃)₂), cobalt nitrate (Co(NO₃)₃), ethoxyethanol, polyvinylpyrrolidone (Mw 55000, 1.1 gram), urea and diethanolamine, were purchased from Fisher Scientific. Hexamethylenetetramine (HMT, 99%) was purchased from Acros. The dilute solution of TiCl₃ (0.15 M) was supplied by Spectrum Chemical MFG. Corp. All chemicals were used as received without further purification. The cordierite and stainless steel monolith substrates were provided by Honda Research Institute (Columbus, Ohio) and Corning Inc. (Corning, N.Y.). The cordierite monolith had 1 mm×1 mm square channels and 100 μm in wall thickness, with a diameter in a range of 1″ to a few inches. The stainless steel monoliths were 1″wide and 1-3″ high.

Characterization

Morphology and structure of the nano-array monolithic catalysts were characterized using a JEOL 6335F Field Emission Scanning Electron Microscope (FESEM) attached with an Energy-Dispersive X-ray Spectrometer (EDXS), a JEOL 2010 FasTEM High-Resolution Transmission Electron Microscope (HRTEM), and a BRUKER AXS D5005 (Cu Kα, 1.540598 Å) X-ray Diffractometer (XRD), at room temperature. The BET surface area and pore size distribution were characterized using Quantachrome NoVA 1000 Gas Sorption Analyzer and Micromeritics ASAP 2020 physisorption analyzer. The thermal stability of samples was investigated using TA instruments SDT Q600 and Q500, and Temperature Programmed Reduction (TPR) under 10 vol. % H₂/N₂ was performed using Micromeritics ChemiSorb 2720. The mass loading of metal oxide nano-array and Pt nanoparticles on monolithic substrates were determined using a Perkin Elmer/DRC-e Inductively Coupled Plasma Mass Spectrometer (ICP-MS) and TEM EDXS.

Growth of TiO₂ Nanorod Array.

Aligned, single-crystalline brookite TiO₂ nanorod arrays were grown on ceramic honeycomb substrate by a low-temperature hydrothermal approach that is very suitable for inexpensive mass production. See E. Hosono, S. Fujihara, K. Kakiuchi, H. Imai, J. Am. Chem. Soc. 2004, 126, 7790. Nanostructured brookite TiO₂ was synthesized by hydrothermal treatment of aqueous titanium trichloride (TiCl₃) solutions with saturated sodium chloride (NaCl). Typically, 20 ml of the precursor solution including 3 g of TiCl₃ and 10 g of DI-water saturated with NaCl was placed in a Teflon-lined autoclave. Either honeycomb cordierite ceramic (or stainless steel) monoliths were then used as substrates and immersed in the solutions. The autoclaves were then heated to 220° C. and maintained for 18 h. After the autoclave was cooled to room temperature, the honeycomb cordierite monoliths were rinsed thoroughly with deionized (DI) water and allowed to dry on a hot plate at 80° C.

Growth of ZnO Nanorod and Nanowire Array.

ZnO nanorod array monolith catalysts were fabricated using methods known in the art modified as described below. See X. Qian, H. Liu, Y. Guo, Song, Y. Y. Li, Nanoscale Res. Lett. 2008, 3, 303. The monolith cordierite was first cleaned by DI water and ethanol in ultrasonic bath and then surface-modified (via dip-coating) with 20 mM zinc acetate ethanol solution for several times to form a seed layer on the channel walls. The dip-coated substrate was then annealed at 350° C. for 5 hours to make the (002) face of ZnO crystal better oriented. After ZnO seed coating, ZnO nanorod growth was accomplished by a classic hydrothermal process. Equal molar zinc nitrate hexahydrate (Zn(NO₃)₂.6H₂O) and hexamethylenetetramine (C₆H₁₂N₄, HMT) (25 mM) were dissolved in 200 mL DI water as precursor. The annealed dip-coated substrate was then placed in the prepared precursor solution at 70-80° C. for 6 hours. The grown ZnO nanorods were rinsed in DI water and sonicated in ethanol for 10 minutes. Finally, the sample was dried at 80° C. in air for further characterization. The length of ZnO nanorods was tuned from 1-5 μm by repeated growth.

Growth of CeO₂ Nanotube Array.

CeO₂ nanotubes array was fabricated by ZnO templated growth. See Y. Chen, T. Tseng, Adv. Sci. Lett. 2008, 1, 123. First, ZnO nanorod array with (002) preferential orientation was prepared as described above for ZnO nanowire array growth. After 2 hr incubation growth of ZnO nanorod arrays at 75° C., cerium nitrate hexahydrate (Ce(NO₃)₃.6H₂O, 125 mM) was then added into the solution, in order to obtain the core/shell-type ZnO/CeO₂ nanorod arrays. Afterward, ZnO/CeO₂ nanorod arrays were immersed in a dilute hydrochloric acid solution (0.05 M) for 40 s at room temperature and then rinsed with DI water to remove residual contaminants such as compounds containing Zn²⁺ or CF in the CeO₂ nanotube arrays. Furthermore, for comparison with the as-grown nanostructure arrays, uncoated ZnO nanorod arrays were also prepared under the same conditions. The length of CeO₂ nanotubes is controlled by the length of ZnO template.

Growth of Co₃O₄ Nanowire Array.

The uniform Co₃O₄ nanowire arrays were obtained from calcinations of hydrothermally prepared cobalt chloride hydroxide carbonate (Co(CO₃)_(0.35)Cl_(0.2)(OH)_(1.1).1.74H₂O). See H. T. Wang, L. Zhang, X. H. Tan, C. M. B. Holt, B. Zahiri, B. C. Olsen, D. Mitlin, J. Phys. Chem. C 2011, 115, 17599. Briefly, 5 mL 0.5 M CoCl₂.6H₂O and 5 mL 6 M urea solution were mixed under ultrasonic bath until the solution became transparent. The pre-cleaned cordierite substrate was then immersed into the as-prepared solution and sonicated for another 15 minutes before being placed into a water bath (90° C.) for 24 hrs. Then the substrate after growth was rinsed with DI water several times and was slowly annealed at 300° C. for 4 hrs.

Synthesis of Pt/Metal Oxide (MO) Nano-Array Catalysts.

The Pt/MO nanorod array catalysts were prepared using a colloidal deposition method. See Y. Wang, J. Ren, K. Deng, L. Gui, Y. Tang, Chem. Mater. 2000, 12, 1622. First, unprotected Pt nanoparticle colloid was synthesized by an ethylene glycol (EG) method, as described in the literature. See Y. Wang, J. Ren, K. Deng, L. Gui, Y. Tang, Chem. Mater. 2000, 12, 1622. Typically, all operations were carried out under inert atmosphere (Ar or N₂). In a typical preparation, a glycol solution of NaOH (50 mL, 0.5 M) was added into a glycol solution of H₂PtCl₆.6H₂O (1.0 g, 1.93 mmol in 50 mL) with stirring to obtain a transparent yellow platinum hydroxide or oxide colloidal solution which was then heated at 160° C. for 3 h, with an Ar flow passing through the reaction system to take away water and organic byproducts. A transparent dark-brown homogeneous colloidal solution of the Pt metal nanocluster (Pt: 3.76 g/L glycol, 19.3 mmol/L) was obtained without any precipitates. For deposition, the as-prepared Pt colloid (Pt/cordierite=0.1 wt %) was diluted by adding ethanol or acetone as solvent to 15-20 mL in different volume. Cordierite monolith substrates with nanostructure arrays were then immersed into the dilute Pt colloid solution for 24 hours with magnetic stirring. Finally, the sample was dried at 80° C. in air, and Pt/MO nano-array monolithic catalysts were ready for use.

Synthesis of ZnO/CeO₂ Nano-Array Catalysts.

ZnO/CeO₂ decorated and core-shell nano-array catalysts were prepared by the same process of CeO₂ nanotube array without etching of ZnO template. ZnO/CeO₂ decorated or core-shell nanostructure can be controlled via CeO₂ deposition time and cycles.

Synthesis of ZnO/(La,Sr)CoO₃ (LSCO), ZnO/LSCO/Pt Nano-Array Catalysts.

The ZnO/LSCO, ZnO/LSCO/Pt nano-array catalysts were prepared by a similar procedure as ZnO/CeO₂ nano-array catalysts. First, a ZnO nanorod array with (002) preferential orientation was uniformly grown on cordierite monolith. Then, by dip coating the colloidal precursor, (La,Sr)CoO₃ or (La,Sr)CoO3/Pt layers were uniformly coated on ZnO nanorod surface throughout the whole substrate. See D. Jian, P.-X. Gao, W. Cai, B. S. Allimi, P S. Alpay, Y. Ding, Z. L. Wang, C. Brooks, J. Mater. Chem. 2009, 19, 970. Finally, the substrate was annealed at 550° C. for 10 hours to ensure good crystallinity and adhesion of the coating.

Morphology and Structure of Various Nanostructure Arrays.

Examples of single component nanostructure arrays are shown in FIGS. 19 and 20.

Two-Component Nanostructure Arrays.

Examples of noble metal-Metal oxide decorated nanostructure arrays are shown in FIGS. 21 and 22.

Examples of metal oxide-metal oxide decorated and core-shell nanostructure arrays are shown in FIG. 23.

Examples of Multi-component nanostructure arrays (ZnO-LSCO—Pt) are shown in FIG. 24.

TABLE 6 The measured BET Specific surface area* of as-prepared nano-array based monolithic catalysts. Blank ZnO- Blank- Stainless TiO₂- Stainless Co₃O₄- Cordierite Steel Cordierite Steel Cordierite Sample Monolith Monolith Monolith Monolith Monolith BET 0.3 2.8 53.9 6.4 8.4 Specific Surface Area (m²/g) *All BET surface areas were calculated including the mass of monolith substrates. Cordierite monolith samples were tested on Micromeritics ASAP 2020 physi-sorption analyzer. Stainless steel sample were performed on Quantachrome NoVA 1000 gas sorption analyzer. Thermal and Mechanical Stability CeO₂ Nanotube Array Monolithic Catalysts: Thermal and Mechanical Stability

TGA-DSC spectra and the corresponding SEM images after thermal analysis experiments on the CeO₂ nanotube array based monolith sample are shown in FIG. 25. After the 1000° C. TGA test, the uniform layer of CeO₂ nanotube array cracked into isolated islands about 10 μm wide individually, which are composed of intact CeO₂ nanotube arrays. However, CeO₂ nanotube array morphology was retained in large scale after 400° C. TGA test. With increasing TGA temperature to 500° C., the nanotube array structure collapsed as a result of a sintering effect, with some of them easily peeled off from the monolith substrate during the sample preparation. These results indicate that the template-induced CeO₂ nanotube array needs to work below 500° C. in order to maintain its mechanical soundness. Compared with the thermal analysis spectra of bare cordierite monolith in FIG. 26 and metal oxide nano-array monolithic catalysts (FIGS. 25 and 28), the small peak on the DSC curve around 100° C. was also observed in all those spectra, which suggest the peak around 100° C. originated from the monolith substrate and not from the metal oxide nano-arrays.

Constant flow flushing tests were performed to test the mechanical (adhesion and structure) stability of CeO₂ nanotube array on cordierite monolith. After flushing for 48 h, serious structural damage was observed (FIG. 27b ), as some of the CeO₂ nanotubes were blown down by air flow and started to aggregate. After continuing flushing for another 48 hours, a large area of bare substrate appeared on the monolith substrate and nearly half of the CeO₂ nanotubes were blown away. After flushing for 168 hours, it was difficult to find CeO₂ nanotube arrays on the cordierite channels and just a few nanotubes left on the monolith surface. The weak adhesion in CeO₂ nanotube arrays to the substrates is due to the template process involved in their growth. Both ZnO seed layer removal and non-uniform distribution of CeO₂ coating on ZnO nanorod array templates have played important roles in drastically reducing the adhesion upon removal of ZnO nanorod templates. However, even though most of the CeO₂ nanotube array has been removed from the monolith substrate by the high flow rate of air, from the mass measurement conducted on the lab general research balance, the weight loss was still zero. This result suggests that the CeO₂ loading on monolith is very small, which is around the measurement limit of lab general balance (1%). This conclusion were also confirmed by the estimation of CeO₂ loading through EDS results of ZnO:CeO₂ weight ratio. The Pt/CeO₂ nano-array monoliths' thermal and mechanical stability could be improved through tubular structure/morphology optimization and structural reinforcement toward improving interface adhesion with monolith substrates. Along this line, direct growth of CeO₂ nanotube arrays or optimization of template growth on monoliths could be the alternative solutions.

Thermal and Mechanical Stability for Other Nanostructure Arrays.

The thermal stability of metal oxide nanostructure-arrays have been tested using thermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques. During the thermal analysis process, there was only less than 0.5% mass loss for TiO₂ nanorods on cordierite monolith (top panel of FIG. 28). After TGA testing, nanorod array morphology and structure was retained both in large scale and in a single nanorod, which suggests that the TiO₂ nanorod array is very stable in the temperature range of 20-1000° C. The ZnO nanorod array also shows the similarly excellent thermal stability on both stainless steel and cordierite monoliths (middle panel of FIG. 28). For Co₃O₄ porous nanowire array (bottom panel of FIG. 28), the total weight loss is about 2.5% in the temperature range of 20-1000° C. The mass loss from 20-250 attributed to the H₂O evaporation. There is a sudden weight loss around 900° C. in the TGA curve and a corresponding endothermic peak at the same temperature in the DSC curve, which suggest that an endothermic transition happened around 900° C. However, from the SEM image after the TGA test, there was no obvious morphology change for Co₃O₄ nanowires array. Given that the melting point and decomposition temperature of Co₃O₄ is around 900° C., there could be a small portion of Co₃O₄ nanowires decomposed or melted in the process, but it had only a minor effect on the nanowire array structure.

Thermal Stability in Reductive Atmosphere: Temperature Programmed Reduction (TPR) Under H₂

Temperature programmed H₂ reduction tests were carried out to evaluate the stability of metal oxide nanostructure arrays in reducing atmosphere. TiO₂ nanorods on cordierite monolith show high stability and inertness in H₂ atmosphere. There was no peak in the range of 20-750° C. and no structural change (as shown in FIGS. 30a and 30c , respectively). ZnO nanorods on monolith substrate started to be reduced at 450° C. with a peak centered at 550° C. The whole reduction window was in a range of between 450° C. and 650° C. TPR spectra of Co₃O₄ porous nanowires array on cordierite monolith and SEM image after TPR are shown in FIGS. 30b and 30c , respectively. There was a very large H₂ consuming peak in a range from 250-550° C. and centered at 350° C., which suggests that a reduction reaction may occur. No obvious structural changes were observed on the nanowire array structure under SEM. However, the XRD after TPR shows a Co peak in the spectrum that confirms that a certain amount of Co₃O₄ was reduced into metal Co in the TPR process. CeO₂ nanotube arrays on monolith substrates were also tested. There was a wide H₂ consuming peak in a range from 300-650° C. and centered at 400° C. and 550° C., respectively. No obvious structure changes were observed on the nanotube array structure under SEM. After TPR-H₂ testing, there seemed to be no ZnO left on the monolith substrate under SEM investigation. All ZnO nanorods were reduced into Zn in TPR process, and the Zn evaporated with the carrier flow in the process. Therefore, ZnO nanorod arrays are stable in reduced atmosphere below 450° C.

Pulsatile Flow and Mechanical (Ultrasonic) Vibration Stability.

A pulsatile flow flushing set up was also introduced for testing the mechanical stability of the as-prepared nanostructure array monolith catalysts. During the test, air flow was switched between 0 and 60 L/min and controlled by a manual switch. 200 cycles were performed in each test. SEM was used to analyze the morphology change in a large scale after experiments. As shown in FIG. 32, all the as-prepared nanostructures exhibited high stability and maintained the array structure and morphology after 200 cycles of highly pulsatile air flow.

On the other hand, directly grown single crystal TiO₂ and ZnO nano-array monolithic catalysts showed great stability and vibration resistance to mechanical vibrations (>1 hr) introduced by Sonicator (42 KHz, 135 W) in both distilled water and ethanol, while Co₃O₄ porous nanowire and CeO₂ nanotube arrays showed some damage due to the polycrystalline nature and non-uniformity of thickness throughout the length of individual nanowire or nanotube.

CO Oxidation

Morphology of Nano-Array Catalysts after CO Oxidation:

The structure of nanostructure array catalysts was characterized after CO oxidation light-off and stability tests. From the SEM images (FIG. 33), it can be seen that crystalline Pt/ZnO and Pt/TiO₂ nanorod array catalysts retained their array structure very well after 3 times repeat run of CO oxidation light-off test and 300° C. 24 hours CO oxidation stability test. However, serious array structural damage was observed on both Pt/Co₃O₄ porous nanowire array catalysts and Pt/CeO₂ nanotube array catalysts as shown in FIG. 33, respectively. As shown in inset TEM images, Pt/Co₃O₄ porous nanowire has turned into solid nanorod structure while the Pt/CeO₂ nanotube structure is still preserved. In summary, after the CO oxidation and stability test, the Pt/CeO₂ nanotube array monolithic catalyst lost its array structure as a whole but preserved the nanotube structure. Both the array structure and the porous nanowire structure were damaged for Pt/Co₃O₄ porous nanowire during the same process. However, the CO oxidation performance of Pt/Co₃O₄ porous nanowire did not decrease during the stability test, but the Pt/CeO₂ nanotube array decreased about 30%, which suggests that, in polycrystalline phase, Pt/Co₃O₄ is not as morphology sensitive as Pt/CeO₂ for the CO oxidation reaction. The results also hold the potential that the activity of as-prepared nanostructure array catalysts can be effectively improved by tuning the morphology and crystal structure.

Pt particle morphology and size distribution change on metal oxide nanostructure array before and after CO oxidation were also characterized by HRTEM. Interestingly, Pt nanoparticles preserve small size and good distribution on metal oxide nanostructure surface. It is widely accepted as particularly difficult to retain stable metal nanoparticles in the size range below 5 nm. It has also been reported that the TTAB-capped Pt on silicon wafer and Pt/MCF exhibited severe aggregation of Pt particles after CO oxidation at 300° C., which hampered the quantitative study of CO oxidation above the ignition temperature regime. See S. H. Joo, J. Y. Park, C.-K. Tsung, Y. Yamada, P. Yang, G. A. Somorjai, Nat. Mater. 2009, 8, 126. However, in this metal oxide supported Pt catalysts system (FIG. 34), Pt nanoparticles still maintained the particle size smaller than 5 nm after 24 h 300° C. CO oxidation stability tests, even though there was a slight size increase (from 2 nm to 3-4 nm for different catalyst systems). Moreover, the catalytic activity of metal oxide/Pt nanostructure array catalysts did not decrease (except CeO₂/Pt) after 24 h 300° C. on stream tests. The metal oxide-Pt interface may play an important role in the stability of as prepared nanostructure array catalysts.

Quantitative Analysis of Materials Usage and Catalytic Performance in Nano-Array Monolithic Catalysts.

Estimated Mass of Pt and Metal Oxide in Each Catalyst.

The mass of metal oxide nanostructure array on cordierite monolith substrate was estimated by a weighing method: M(MO)=M₁−M₀ where M₀ is the mass of blank monolith before metal oxide nanostructure growth; M₁ is the mass of monolith substrate after metal oxide nanostructure growth. Mass of Pt was quantified by using statistical TEM EDS element ratio analysis results as reference: M(Pt)=wt % (Pt/MO)×M(MO)

Wt % (Pt/MO) were obtained from statistical results of TEM EDXS analysis.

Co₃O₄ Porous Nanowires Array on Cordierite Monolith:

By measuring the mass difference before and after Co₃O₄ porous nanowires array loading, Co₃O₄ porous nanowires loaded on cordierite monolithic catalysts take 10% of the weight (statistical results) in the monolithic catalysts. The sample used for catalytic performance test was about 38.3 mg. Then the mass of Co₃O₄ porous nanowires array was about 3.83 mg.

Pt loading was controlled with Pt:Co₃O₄ weight ratio of 1% by controlling the total amount of Pt colloidal solution as 1 wt % of Co₃O₄ nano-array on monolith. Moreover, TEM EDXS results also confirmed the 1% weight ratio between Pt and Co₃O₄. As a result, Pt loaded on Co₃O₄ porous nanowires array should be no more than 0.0383 mg.

ZnO Nanorod Array on Cordierite Monolith:

The weight percentage of seed nanoparticles with respect to total mass of the dip-coated substrate was determined by the weighing method to be (1.57±0.06) %. To reduce the influence of substrate destroyed during the growing process, the weight loss was measured after etching of ZnO by using 50 mM HCl solution for 1 min. The weight percentage of deposited ZnO NR arrays with respect to total mass of the grown substrate was determined to be (3.8±0.2) %. Pt/ZnO nanorod array monolithic catalyst used in the test was 37.17 mg in mass. Then, the amount of ZnO nanorods on it was about 1.484 mg.

The same amount of Pt colloidal solution and loading procedure as Pt/Co₃O₄ loading were used for Pt/ZnO preparation. By TEM EDX analysis, the average weight ratio between Pt and ZnO was 2% (from statistical results), then the mass of Pt in ZnO nano-array monolithic catalysts was about 0.030 mg.

CeO₂ Nanotubes Array on Cordierite Monolith:

The mass of CeO₂ nanotubes loading on cordierite monolith was estimated according to the weight ratio of ZnO:CeO₂ in the composite nanostructure array from SEM EDXS analysis. The average weight ratio of ZnO:CeO₂ is about 3.53:1. The 1 μm ZnO nanorods loading amount is typically about 4% of the total monolith catalysts. Therefore, the loading of CeO₂ on cordierite monolith was about 1% of the whole monolith catalysts.

Pt/CeO₂ nanotubes array monolithic catalyst used in the test was 31.7 mg in mass. Then, the amount of CeO₂ nanotubes on it was about 0.317 mg.

The same amount of Pt and the same procedure was used for Pt/CeO₂ catalysts preparation as for Pt/ZnO. TEM EDX analysis results show that the average weight ratio between Pt and CeO₂ was 10% (from statistical results), so the mass of Pt in the monolithic catalysts was about 0.0317 mg.

TiO₂ Nanorod Array on Cordierite Monolith:

The mass of TiO₂ nanorods loading on cordierite monolith was estimated according to the law of mass conservation. Assuming that all Ti precursor 100% converted to TiO₂, the total mass of TiO₂ should be 250 mg. Actually, not all of the TiO₂ was loaded on the monolith substrate. There were TiO₂ precipitates on the bottom of the reactor, which were collected, dried and weighed. The result was about 203.5 mg in mass. There were 46.5 mg TiO₂ on 3.06 g cordierite monolith. Then, there were 0.4463 mg TiO₂ nanorod array loaded on 29.4 mg monolithic catalysts (about 6% of the total monolithic catalysts mass), which were used for the CO oxidation tests.

The same amount of Pt and the same procedure as applied to the above three catalysts were used for Pt/TiO₂ nano-array catalysts synthesis. As the statistical results of weight ratio between Pt and TiO₂ were 8% (from statistical results from TEM analysis), the mass of Pt in the monolithic catalysts was about 0.0357 mg.

Compared to Pt/ZnO and Pt/TiO₂ nanorod array catalysts, Pt/CeO₂ nanotube array and Pt/Co₃O₄ porous nanowire array exhibit higher CO oxidation performance, which is due to the promotion effect of Co₃O₄ and CeO₂ to Pt nanoparticles. In both Pt-loaded Co₃O₄ and CeO₂ catalytic systems, CO molecules mainly adsorb on the Pt sites, whereas O₂ could be more favorably adsorbed and activated on the cerium or cobalt sites, which may effectively improve the O₂ adsorption on supported catalysts and thus increase the catalytic activity. Besides, metal-metal oxide interactions may help enhance the catalytic activity as well. In Pt/ZnO and Pt/TiO₂ nanorod array monolithic catalysts, similar CO catalytic oxidation performance was achieved, which follows a Langmuir-Hinshelwood mechanism that both CO and O₂ adsorb on the Pt surface itself.

Statistical Data of Catalytic Performance and Materials Usage/Loading of Nano-Array Catalysts and Powder-Form Catalysts.

TABLE 7 Catalytic performance data of different metal oxide nano-array monolithic catalysts. K 50% 100% Mass of (Catalysts Light-Off Conversion Conversion Loading Activity) Temperature Temperature Temperature Pt/MO at 230° C. Catalysts (T₁₀) (° C.) (T₅₀) (° C.) (T₁₀₀) (° C.) (mg/mg)* (mol/g_(pt) · s) 1 μm 234 260 267 0.011/1.148 0.18 Pt/ZnO 5 μm 220 234 236 0.011/ 0.78 Pt/ZnO Pt/CeO₂ 165 193 201 0.054/0.317 0.41 Pt/Co₃O₄ 168 195 205 0.038/3.83 0.58 Pt/TiO₂ 245 258 269 0.094/1.148 1.4 × 10⁻² *Loading mass of Pt were determined by using statistical TEM EDXS element ratio between Pt and Metal oxide

During the isothermal CO oxidation test, the amount of Pt/CeO₂ activity site decreased along with the decrease of surface area caused by sintering effect, in which process CeO₂ nanotubes aggregated to each other leading to the loss of the original aligned array structure. SEM characterization on the Pt/CeO₂ nanotube array catalysts sample after CO oxidation and catalytic stability test showed that serious damage of nanotube array structure had happened to the catalysts and left a Pt/CeO₂ film on the surface of the cordierite monolith channel. Even though the CeO₂ nanotube was not seriously destroyed during the CO reaction, the Pt particle size increased slightly, which may lead to the decrease of catalytic activity.

TABLE 8 Catalytic performance data of various ZnO nano-array (nanorod (NR) nanoplate (NP)), and washcoated ZnO nanopowder monolithic catalysts Pt K (catalysts loading activity, Ratio 230° C., Catalyst T₁₀ (° C.) T₅₀ (° C.) T₁₀₀ (° C.) (g/L)* mol/g_(pt) · S) 1 μm Pt/ZnO 230 260 267 0.49 1.1 × 10⁻³ NR 5 μm Pt/ZnO 220 234 236 0.51 4.1 × 10⁻³ NR Pt/ZnO 195 235 242 0.28 5.8 × 10⁻³ NP Pt/ZnO 260 280 290 0.37 5.6 × 10⁻⁴ Washcoated Powder

Washcoated ZnO powder monolithic catalysts were prepared by dip-coating ZnO powder directly purchased from Thermo-Fisher Company. The loading on ZnO washcoated powder were monitored to around 5 μm in thickness by SEM observation. Pt loading was conducted exactly the same parameter and process as Pt/ZnO nano-array catalysts.

The relevant teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety.

While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims. 

What is claimed is:
 1. A metal oxide nanorod array structure comprising: a honeycomb monolithic substrate having a surface and multiple channels; an interface layer bonded to the surface of the substrate; and a metal oxide nanorod array coupled to the substrate surface via the interface layer, wherein the metal oxide nanorod is formed of TiO₂, CeO₂, ZnO, SnO₂, Al₂O₃, ZrO₂, Co₃O₄, or Ga₂O₃.
 2. The nanorod array structure of claim 1, wherein the metal oxide nanorod array is vertically aligned relative to the surface of the substrate.
 3. The nanorod array structure of claim 2, wherein the metal oxide nanorod is formed of titania (TiO₂).
 4. The nanorod array structure of claim 3, wherein the titania is brookite titania.
 5. The nanorod array structure of claim 1, wherein the substrate is formed of plastic, silicon, ceramic or stainless steel.
 6. The nanorod array structure of claim 4, wherein the substrate is formed of ceramic.
 7. The nanorod array structure of claim 6, wherein the ceramic includes one of the following: cordierite, alumina, tin oxide, and titania.
 8. The nanorod array structure of claim 1, wherein the metal oxide nanorod includes one of the following: ceria (CeO₂), zinc oxide (ZnO), tin oxide (SnO2), alumina (Al₂O₃), zirconia (ZrO₂), cobalt oxide (Co₃O₄), and gallium oxide (Ga₂O₃).
 9. The nanorod array structure of claim 1, further including a perovskite ABO₃ (A=La, Ba, Sr, and/or Pb, B=Co, Cr, Fe, Mn, or Ni) shell over each metal oxide nanorod.
 10. The nanorod array structure of claim 9, wherein the perovskite is a lanthanum-based transition metal oxide, LaMO₃ (M=Co, Cr, Fe, Mn, and Ni).
 11. The nanorod array structure of claim 10, wherein the lanthanum-based transition metal oxide is lanthanum strontium cobaltate.
 12. The nanorod array structure of claim 10, wherein the lanthanum-based transition metal oxide is lanthanum strontium manganate.
 13. The nanorod array structure of claim 1, further including a metal oxide shell over each metal oxide nanorod.
 14. The nanorod array structure of claim 13, wherein the shell includes one of the following metal oxides: ceria (CeO₂), zinc oxide (ZnO), tin oxide (SnO₂), alumina (Al₂O₃), zirconia (ZrO₂), cobalt oxide (CO₃O₄), and gallium oxide (Ga₂O₃).
 15. The nanorod array structure of claim 1, further including an alkaline oxide shell over each metal oxide nanorod.
 16. The nanorod array structure of claim 15, wherein the alkaline oxide shell includes at least one of the following oxides: barium oxide and potassium oxide.
 17. The nanorod array structure of claim 1, further including a coating of metal particles over each metal oxide nanorod, the metal including at least one of the following: platinum, gold, palladium, rhodium, and ruthenium.
 18. The nanorod array structure of claim 9, further including a coating of metal particles over each metal oxide nanorod, the metal including at least one of the following: platinum, gold, palladium, rhodium, and ruthenium. 